Changes in the Zend Engine 2.0

    * New Object Model.

      The Zend Engine's handling of objects has been completely
      changed in order to allow for new features, but also to increase
      its performance.

      Objects were handled in previous versions like primitive types
      (for instance integers and strings). The drawback of this method
      is, that semantically the whole object was copied when a
      variable was assigned or parameters were passed to a method. The
      new approach refers to objects by handle and not by value (one
      can think of a handle as an object's ID).

      Many PHP programmers aren't even aware of the copying quirks of
      the old object model and, therefore, there is a relatively good
      chance that the amount of PHP applications that will work out of
      the box or after a very small amount of modifications would be
      high.

    * $this

      Unlike in Zend Engine 1 the pseudo variable $this cannot be
      exchanged in Zend Engine 2. You can of course modify or work with
      an object by using $this but you cannot replace $this with another
      object to change the original object.

      Example:

        <?php
        class Foo {
            function replace($other)
            {
                $this = $other;
            }
        }

        $object = new Foo;
        $object->prop = 'Hello';

        $other  = new Foo;
        $other->prop = 'Bye';

        $object->replace($other);

        print $object->prop; // still shows 'Hello'

        ?>

        Zend Engine 2.0 will issue a compile error, if an assignment
        to $this is found.

    * Private and Protected Members.

      The Zend Engine 2.0 introduces private and protected member
      variables. Note that for performance reasons no error message is
      emitted in case of an illegal access to a private or protectecd
      member variable.

      Example:

        <?php
        class MyClass {
            private $Hello = "Hello, World!\n";
            protected $Bar = "Hello, Foo!\n";
            protected $Foo = "Hello, Bar!\n";

            function printHello() {
                print "MyClass::printHello() " . $this->Hello;
                print "MyClass::printHello() " . $this->Bar;
                print "MyClass::printHello() " . $this->Foo;
            }
        }

        class MyClass2 extends MyClass {
            protected $Foo;

            function printHello() {
                MyClass::printHello();                          /* Should print */
                print "MyClass2::printHello() " . $this->Hello; /* Shouldn't print out anything */
                print "MyClass2::printHello() " . $this->Bar;   /* Shouldn't print (not declared)*/
                print "MyClass2::printHello() " . $this->Foo;   /* Should print */
            }
        }

        $obj = new MyClass();
        print $obj->Hello;  /* Shouldn't print out anything */
        print $obj->Bar;    /* Shouldn't print out anything */
        print $obj->Foo;    /* Shouldn't print out anything */
        $obj->printHello(); /* Should print */

        $obj = new MyClass2();
        print $obj->Hello;  /* Shouldn't print out anything */
        print $obj->Bar;    /* Shouldn't print out anything */
        print $obj->Foo;    /* Shouldn't print out anything */
        $obj->printHello();
        ?>

      Protected member variables can be accessed in classes extending the
      class they are declared in, whereas private member variables can
      only be accessed by the class they belong to.

    * Private and protected methods.

      The Zend Engine 2.0 introduces private and protected methods.

      Example:

        <?php
        class Foo {
          private function aPrivateMethod() {
            echo "Foo::aPrivateMethod() called.\n";
          }

          protected function aProtectedMethod() {
            echo "Foo::aProtectedMethod() called.\n";
            $this->aPrivateMethod();
          }
        }

        class Bar extends Foo {
          public function aPublicMethod() {
            echo "Bar::aPublicMethod() called.\n";
            $this->aProtectedMethod();
          }
        }

        $o = new Bar;
        $o->aPublicMethod();
        ?>

      Old code that has no user-defined classes or functions named
      'public', 'protected' or 'private' should run without modifications.

    * Abstract Classes and Methods.

      The Zend Engine 2.0 introduces abstract classes and methods. An
      abstract method only declares the method's signature and does not
      provide an implementation. A class that contains abstract methods
      needs to be declared abstract.

      Example:

        <?php
        abstract class AbstractClass {
          abstract public function test();
        }

        class ImplementedClass extends AbstractClass {
          public function test() {
            echo "ImplementedClass::test() called.\n";
          }
        }

        $o = new ImplementedClass;
        $o->test();
        ?>

      Classes that do not have abstract methods can be declared abstract
      to prevent them from being instantiated.

      Old code that has no user-defined classes or functions named
      'abstract' should run without modifications.

    * Interfaces.

      The Zend Engine 2.0 introduces interfaces. A class may implement
      an arbitrary list of interfaces.

      Example:

        <?php
        interface Printable {
          public function dump();
        }

        class PrintableExample implements Printable {
          public function dump() {
            // ...
          }
        }
        ?>

      Old code that has no user-defined classes or functions named
      'interface' or 'implements' should run without modifications.

      An interface may extend one or more base interfaces (but not
      implement them). Neither a class nor an interface can inherit
      methods of the same name from different root interfaces.

      Interfaces may contain abstract static methods.

      Example:

        <?php
        interface Printable {
          function dump();
        }

        interface Streamable extends Printable {
          function writeToStream();
          static function readFromStream();
        }

        class PrintableExample implements Streamable {
          public function dump() {
            // ...
          }
          function writeToStream() {
            // ...
          }
          static function readFromStream() {
            // ...
          }
        }
        ?>

      A class that does not implement all interface methods must be
      declared as an abstract class.

    * Class Type Hints.

      While remaining loosely typed the Zend Engine 2.0 introduces the
      ability to use class type hints to declare the expected class of
      objects that are passed as parameters to a method.

      Example:

        <?php
        interface Foo {
          function a(Foo $foo);
        }

        interface Bar {
          function b(Bar $bar);
        }

        class FooBar implements Foo, Bar {
          function a(Foo $foo) {
            // ...
          }

          function b(Bar $bar) {
            // ...
          }
        }

        $a = new FooBar;
        $b = new FooBar;

        $a->a($b);
        $a->b($b);
        ?>

      These class type hints are not checked upon compilation, as would
      be the case in a typed language, but during runtime.

      This means that

        function foo(ClassName $object) {
          // ...
        }

      is equivalent to

        function foo($object) {
          if (!($object instanceof ClassName)) {
            die('Argument 1 must be an instance of ClassName');
          }
        }

      This syntax only applies to objects/classes, not built-in types.

    * Final methods and classes.

      The Zend Engine 2.0 introduces the "final" keyword to declare
      final methods. Those cannot be overridden by sub-classes.

      Example:

        <?php
        class Foo {
          final function bar() {
            // ...
          }
        }
        ?>

      It is furthermore possible to make a class final. Doing this
      prevents a class from being specialized (it cannot be inherited
      by another class). There's no need to declare the methods of
      a final class themselves as final.

      Example:

        <?php
         final class Foo {
          // class definition
        }
        // the next line is impossible
        // class Bork extends Foo {}
        ?>

      Properties cannot be final. See per-class constants below.

      Old code that has no user-defined classes or functions named
      'final' should run without modifications.

    * Object Cloning.

      The Zend Engine 1.0 offered no way a user could decide what copy
      constructor to run when an object is duplicated. During
      duplication, the Zend Engine 1.0 did a bitwise copy making an
      identical replica of all the object's properties.

      Creating a copy of an object with fully replicated properties is
      not always the wanted behavior. A good example of the need for
      copy constructors, is if you have an object which represents a
      GTK window and the object holds the resource of this GTK window,
      when you create a duplicate you might want to create a new
      window with the same properties and have the new object hold the
      resource of the new window. Another example is if your object
      holds a reference to another object which it uses and when you
      replicate the parent object you want to create a new instance of
      this other object so that the replica has its own separate copy.

      An object copy is created by using the clone operator.

      Example:

        <?php
        $copy_of_object = clone $object;
        ?>

      When the developer asks to create a new copy of an object, the
      Zend Engine will check if a __clone() method has been defined or
      not. If not, it will call a default __clone() which will copy
      all of the object's properties. If a __clone() method is
      defined, then it will be responsible to set the necessary
      properties in the created object. For convenience, the engine
      ensures, that the clone will be initialized with all of the
      properties from the source object, so that developers can start
      with a by-value replica of the source object, and only override
      properties that need to be changed.

      Example:
        <?php
        class MyCloneable {
          static $id = 0;

          function MyCloneable() {
            $this->id = self::$id++;
          }

          function __clone() {
            $this->address = 'New York';
            $this->id = self::$id++;
          }
        }

        $obj = new MyCloneable();

        $obj->name = 'Hello';
        $obj->address = 'Tel-Aviv';

        $obj_clone = clone $obj;

        print $obj->id . "\n";
        print $obj->name . "\n";
        print $obj->address . "\n";

        print $obj_clone->id . "\n";
        print $obj_clone->name . "\n";
        print $obj_clone->address . "\n";
        ?>

    * Unified Constructors.

      The Zend Engine allows developers to declare constructor methods
      for classes. Classes which have a constructor method call this
      method on each newly-created object, so it is suitable for any
      initialization that the object may need before it can be used.

      With the Zend Engine 1.0, constructor methods were class methods
      that had the same name as the class itself. Since it is very
      common to call parent constructors from derived classes, the way
      the Zend Engine 1.0 worked made it a bit cumbersome to move
      classes around in a large class hierarchy. If a class is moved
      to reside under a different parent, the constructor name of that
      parent changes as well, and the code in the derived class that
      calls the parent constructor has to be modified.

      The Zend Engine 2.0 introduces a standard way of declaring
      constructor methods by calling them by the name __construct().

      Example:

        <?php
        class BaseClass {
            function __construct() {
                print "In BaseClass constructor\n";
            }
        }

        class SubClass extends BaseClass {
            function __construct() {
                parent::__construct();
                print "In SubClass constructor\n";
            }
        }

        $obj = new BaseClass();
        $obj = new SubClass();
        ?>

      For backwards compatibility, if the Zend Engine 2.0 cannot find
      a __construct() function for a given class, it will search for
      the old-style constructor function, by the name of the class.
      Effectively, it means that the only case that would have
      compatibility issues is if the class had a method named
      __construct() which was used for different semantics.

    * Destructors.

      Having the ability to define destructors for objects can be very
      useful. Destructors can log messages for debugging, close
      database connections and do other clean-up work.

      No mechanism for object destructors existed in the Zend Engine
      1.0, although PHP had already support for registering functions
      which should be run on request shutdown.

      The Zend Engine 2.0 introduces a destructor concept similar to
      that of other object-oriented languages, such as Java: When the
      last reference to an object is destroyed the object's
      destructor, which is a class method name __destruct() that
      recieves no parameters, is called before the object is freed
      from memory.

      Example:

        <?php
        class MyDestructableClass {
            function __construct() {
                print "In constructor\n";
                $this->name = 'MyDestructableClass';
            }

            function __destruct() {
                print 'Destroying ' . $this->name . "\n";
            }
        }

        $obj = new MyDestructableClass();
        ?>

      Like constructors, parent destructors will not be called
      implicitly by the engine. In order to run a parent destructor,
      one would have to explicitly call parent::__destruct() in the
      destructor body.

    * Constants.

      The Zend Engine 2.0 introduces per-class constants.

      Example:

        <?php
        class Foo {
            const constant = 'constant';
        }

        echo 'Foo::constant = ' . Foo::constant . "\n";
        ?>

      Old code that has no user-defined classes or functions
      named 'const' will run without modifications.

    * Exceptions.

      The Zend Engine 1.0 had no exception handling. The Zend Engine 2.0
      introduces a exception model similar to that of other programming
      languages. But there is no catch all and no finally clause.

      Old code that has no user-defined classes or functions 'catch',
      'throw' and 'try' will run without modifications.

      Exceptions can be rethrown in catch blocks. Also it is possible to
      have multiple catch blocks. In that case the caught exception is
      compared with the classtype of each catch block from top to bottom
      and the first block that has a 'instanceof' match gets executed.
      When the catch block finishes execution continues at the end of
      the last catch block. If no catch block has a 'instanceof' match
      then the next try/catch block is searched until no more try/catch
      blocks are available. In that case the exception is an uncaught
      exception and the program terminates with showing the exception.

      Example:

        <?php
        class MyException {
            function __construct($exception) {
                $this->exception = $exception;
            }

            function Display() {
                print "MyException: $this->exception\n";
            }
        }

        class MyExceptionFoo extends MyException {
            function __construct($exception) {
                $this->exception = $exception;
            }

            function Display() {
                print "MyException: $this->exception\n";
            }
        }

        try {
            throw new MyExceptionFoo('Hello');
        }
        catch (MyException $exception) {
            $exception->Display();
        }
        catch (Exception $exception) {
            echo $exception;
        }
        ?>

      Even though the above example shows that it is possible to define
      exception classes  that don't inherit from Exception it is best to
      do so. This is because the internal Exception class can gather a
      lot of information otherwise not available. The PHP code emulation
      code would look something like shown below. The comments show the
      meaning of each property. As the code shows it is possible to read
      any available information by using the getter methods. But since
      some of the methods are used internally they are marked final. All
      in all the class is very restrictive because it must be ensured
      that anything used internally always  works as expected.

      Emulating class Exception:

        <?php
        class Exception {
          function __construct(/*string*/ $message=NULL, /*int*/ $code=0) {
            if (func_num_args()) {
              $this->message = $message;
            }
            $this->code = $code;
            $this->file = __FILE__; // of throw clause
            $this->line = __LINE__; // of throw clause
            $this->trace = debug_backtrace();
            $this->string = StringFormat($this);
          }

          protected $message = 'Unknown exception';  // exception message
          protected $code = 0; // user defined exception code
          protected $file;     // source filename of exception
          protected $line;     // source line of exception

          private $trace;      // backtrace of exception
          private $string;     // internal only!!

          final function getMessage() {
            return $this->message;
          }
          final function getCode() {
            return $this->code;
          }
          final function getFile() {
            return $this->file;
          }
          final function getTrace() {
            return $this->trace;
          }
          final function getTraceAsString() {
            return self::TraceFormat($this);
          }
          function _toString() {
            return $this->string;
          }
          static private function StringFormat(Exception $exception) {
            // ... a function not available in PHP scripts
            // that returns all relevant information as a string
          }
          static private function TraceFormat(Exception $exception) {
            // ... a function not available in PHP scripts
            // that returns the backtrace as a string
          }
        }
        ?>

      If you derive your exception classes from this Exception base class
      your exceptions will be nicely shown in the builtin handler for
      uncaught exceptions.

      Note: The method getMessage() is a final read only access method to
      the private proeprty message that is set in the constructor. If you
      feel a need to overwrite the exception display then overload method
      __toString() in your derived class or implement your own extneral
      exception display function to accomplish your desired formatting.

      Example:

      <?php
      function display_exception(Exception $ex)
      {
          echo 'Exception: ' . $ex->getMessage() . 'with backtrace: <pre>';
          echo $ex->getTrace();
          echo '</pre>';
      }

      try
      {
          // your code here
      }
      catch (Exception $ex)
      {
          display_exeption($ex);
      }
      ?>

    * Dereferencing objects returned from functions.

      Example:

        <?php
        class Circle {
            function draw() {
                print "Circle\n";
            }
        }

        class Square {
            function draw() {
                print "Square\n";
            }
        }

        function ShapeFactoryMethod($shape) {
            switch ($shape) {
                case 'Circle': return new Circle();
                case 'Square': return new Square();
            }
        }

        ShapeFactoryMethod('Circle')->draw();
        ShapeFactoryMethod('Square')->draw();
        ?>

    * Member variables of classes can now be initialized.

      Example:

        <?php
        class foo {
            static $my_static = 5;
            public $my_prop = 'bla';
        }

        print foo::$my_static;

        $obj = foo;

        print $obj->my_prop;
        ?>

    * Static Methods.

      The Zend Engine 2.0 introduces the 'static' keyword to declare
      a method static, thus callable from outside the object context.

      Example:

        <?php
        class Foo {
          public static function aStaticMethod() {
            // ...
          }
        }

        Foo::aStaticMethod();
        ?>

      The pseudo variable $this is not available inside a method that
      has been declared static.

    * instanceof.
      New support for an instanceof operator which checks if an object
      is of a certain class or interface type.

      Example:

      <?php

      class Foo {
      }

      $obj = new Foo();
      if ($obj instanceof Foo) {
        print "Yay!\n";
      }
      ?>

    * Parameters that are passed by reference to a function
      may now have default values.

      Example:

        <?php
        function my_function(&$var = null) {
             if ($var === null) {
                 die('$var needs to have a value');
             }
        }
        ?>

    * __autoload().

      The __autoload() interceptor function will be automatically called
      when an undeclared class is to be instantiated. The name of that
      class will be passed to the __autoload() interceptor function as its
      only argument. __autoload() must succeed in loading the class. If it
      doesn't then an E_ERROR is emitted.

      Example:

        <?php
        function __autoload($className) {
          include_once $className . '.php';
        }

        $object = new ClassName;
        ?>

    * Method calls and property accesses can be overloaded
      by class methods  __call(), __get() and __set().

      __get() and __set() Example:

        <?php
            class Setter {
                public $n;
                public $x = array('a' => 1, 'b' => 2, 'c' => 3);

                function __get($nm) {
                    print "Getting [$nm]\n";

                    if(isset($this->x[$nm])) {
                        $r = $this->x[$nm];
                        print "Returning: $r\n";
                        return $r;
                    } else {
                        print "Nothing!\n";
                    }
                }

                function __set($nm, $val) {
                    print "Setting [$nm] to $val\n";

                    if(isset($this->x[$nm])) {
                        $this->x[$nm] = $val;
                        print "OK!\n";
                    } else {
                        print "Not OK!\n";
                    }
                }
            }

            $foo = new Setter();
            $foo->n = 1;
            $foo->a = 100;
            $foo->a++;
            $foo->z++;
            var_dump($foo);
        ?>

      __call() Example:

        <?php
            class Caller {
                var $x = array(1, 2, 3);

                function __call($m, $a) {
                     print "Method $m called:\n";
                     var_dump($a);
                     return $this->x;
                }
            }

            $foo = new Caller();
            $a = $foo->test(1, '2', 3.4, true);
            var_dump($a);
        ?>

    * Iteration

      Objects may be iterated in an overloaded way when used with
      foreach. The default behavior is to iterate over all properties
      with respect to property visibility.

      Example:

        <?php
            class Foo {
                var $x = 1;
                var $y = 2;
            }

            $obj = new Foo;

            foreach ($obj as $prp_name => $prop_value) {
                // using the property
            }
        ?>

      Each class whose instances can be iterated with foreach should
      implement the empty interface 'Traversable'. Hence any object
      that says it implements 'Traversable' can be used with foreach.

      The interfaces 'IteratorAggregate' and 'Iterator' allow to specify
      how class objects are iterated in PHP code. The first of them simply
      has a method 'getIterator' which must return an object that either
      implements the interface 'Iterator' or is instantiated from an
      internal class that can be iterated.

      Example:

        <?php
            class ObjectIterator implements Iterator {

                private $obj;
                private $num;

                function __construct($obj) {
                    $this->obj = $obj;
                }
                function rewind() {
                    $this->num = 0;
                }
                function valid() {
                    return $this->num < $this->obj->max;
                }
                function key() {
                    return $this->num;
                }
                function current() {
                    switch($this->num) {
                        case 0: return "1st";
                        case 1: return "2nd";
                        case 2: return "3rd";
                        default: return $this->num."th";
                    }
                }
                function next() {
                    $this->num++;
                }
            }

            class Object implements IteratorAggregate {

                public $max = 3;

                function getIterator() {
                    return new ObjectIterator($this);
                }
            }

            $obj = new Object;

            // this foreach ...
            foreach($obj as $key => $val) {
                echo "$key = $val\n";
            }

            // matches the following 7 lines with the for directive.
            $it = $obj->getIterator();
            for($it->rewind(); $it->valid(); $it->next()) {
                $key = $it->key();
                $val = $it->current();
                echo "$key = $val\n";
            }
            unset($it);
        ?>

      The matching for directive is very intersting here since it shows
      the use of all abstract methods declared in the interfaces Iterator
      and IteratorAggregate respectively.

    * Array overloading
    
      Objects can be used with Array notation when they implement the 
      interface ArrayAccess. You cannot use such objects in standard
      array functions, however you have full control over the array
      notation. This allows lazy initialization or read only array.
      
      Note that setting [] results in a call to offsetSet() with
      index being NULL. That means that as with standard arrays you 
      cannot store NULL keys.
      
      Example:
      
        <?php
	        class ArrayClass implements ArrayAccess {
				public $a = array();
			
				function offsetExists($index) {
					return array_key_exists($index, $this->a);
				}
				function offsetGet($index) {
					return $this->a[$index];
				}
				function offsetSet($index, $newval) {
					return $this->a[$index] = $newval;
				}
				function offsetUnset($index) {
					unset($this->a[$index]);
				}
			}
			
			$obj = new ArrayClass;
			
			$obj[0] = 'bla';     // calls offsetSet(0,'bla')
			$obj[] = 42;         // calls offsetSet(NULL, 42)
			$x = $obj[0];        // calls offsetGet(0)
			$b = isset($obj[0]); // calls offsetExists(0)
			unset($obj[0]);      // calls offsetUnset(0)
		?>
 

    * __METHOD__

      The pseudo constant __METHOD__ shows the current class and method
      when used inside a method and the function when used outside of a
      class.

      Example:

        <?php
            class Foo {
                function Show() {
                    echo __FILE__ . '(' . __LINE__ . ')' . __METHOD__;
                }
            }
            function Test() {
                echo __FILE__ . '(' . __LINE__ . ')' . __METHOD__;
            }
        ?>

    * __toString()

      The magic method __toString() allows to overload the object to
      string conversion. This conversion is only done automatically for
      the printing functions (echo, print) but not for other functions
      that expect strings. Also the function __toString is not used in
      places where objects are not allowed but strings are like array
      indices. Note that specialized objects may be converted to a string
      in any place but without calling __toString().

      Example:

        <?php
            class Foo {
                function __toString() {
                    return "What ever";
                }
            }

            $obj = new Foo;
            
            $str = (string) $obj; // call __toString()
            
            echo $obj; // call __toString()
            
            $ar = array();
            $ar[(string)$obj]; // this works
            $ar[$obj];         // this is not allowed
        ?>

    * Reflection API

      PHP 5 comes with a complete Reflection API that adds the ability to
      reverse-engineer classes, interfaces, functions and methods as well
      as extensions.

      The Reflection API also offers ways of getting doc comments for
      functions, classes and methods.

      Nearly all aspects of object oriented code can be reflected by
      using the Reflection API which is documented separately:
      http://sitten-polizei.de/php/reflection_api/docs/language.reflection.html

      Example:

        <?php
            class Foo {
                public $prop;
                function Func($name) {
                    echo "Hello $name";
                }
            }

            ReflectionClass::export('Foo');
            ReflectionObject::export(new Foo);
            ReflectionMethod::export('Foo', 'func');
            ReflectionProperty::export('Foo', 'prop');
            ReflectionExtension::export('standard');
        ?>

    * New memory manager
      The Zend Engine has a new memory manager which allows it to run efficiently
      in multi-threaded environments as it doesn't need to use mutexes to lock
      and unlock during allocation/deallocation.

    * Others
      Probably other changes which we forgot to list. This list will be kept up-to-date
      as much as possible.


Changes in the Zend Engine 1.0

  The Zend Engine was designed from the ground up for increased speed,
  reduced memory consumption and more reliable execution. We dare say
  it meets all of these goals and does so pretty well. Beyond that,
  there are several improvements in the language engine features:

    * References support.

      $foo = &$a; would make $foo and $a be two names to the same
      variable. This works with arrays as well, on either side; e.g.,
      $foo = &$a[7]; would make $foo and $a[7] be two names to the
      same variable. Changing one would change the other and vice
      versa.

    * Object overloading support.

      This feature allows various OO libraries to use the OO notation
      of PHP to access their functionality. Right now, no use is made
      of that feature, but we'd have a COM module ready by the time
      PHP 4.0 is released. A CORBA module would probably follow.

    * include() and eval() are now functions, and not statements.

      That means they return a value. The default return value from
      include() and eval() is 1, so that you can do if (include())
      without further coding. The return value may be changed by
      returning a value from the global scope of the included file or
      the evaluated string. For example, if 'return 7;' is executed in
      the global scope of foo.inc, include('foo.inc') would evaluate
      to 7.

    * Automatic resource deallocation.

      Several people have been bitten by the fact that PHP 3.0 had no
      concept of reference counting. The Zend Engine adds full
      reference counting for every value in the system, including
      resources. As soon as a resource is no longer referenced from
      any variable, it is automatically destroyed to save memory and
      resources. The most obvious example for the advantage in this is
      a loop that has an SQL query inside it, something like '$result
      = sql_query(...);'. In PHP 3.0, every iteration resulted in
      another SQL result-set allocated in the memory, and all of the
      result sets weren't destroyed until the end of the script's
      execution. With the Zend Engine, as soon as we overwrite an old
      result set with a new one, the old result set which is no longer
      referenced, is destroyed.

    * Full support for nesting arrays and objects within each other,
      in as many levels as you want.

    * true and false are now constants of type boolean.

      Comparing any other value to them would convert that value to a
      boolean first, and conduct the comparison later. That means, for
      example, that 5==true would evaluate to true (in PHP 3.0, true
      was nothing but a constant for the integer value of 1, so
      5==true was identical to 5==1, which was false).

    * Runtime binding of function names.

      This complex name has a simple explanation - you can now call
      functions before they're declared!

    * Added here-docs support.

    * Added foreach.

      Two syntaxes supported:

        foreach(array_expr as $val) statement
        foreach(array_expr as $key => $val) statement

    * A true unset() implementation.

      A variable or element that is unset(), is now sent to oblivion
      in its entirely, no trace remains from it.

    * Output buffering support.

      Use ob_start() to begin output buffering, ob_end_flush() to end
      buffering and send out the buffered contents, ob_end_clean() to
      end buffering without sending the buffered contents, and
      ob_get_contents() to retreive the current contents of the output
      buffer. Header information (header(), content type, cookies) are
      not buffered. By turning on output buffering, you can
      effectively send header information all throughout your file,
      regardless of whether you've emitted body output or not.

    * Full variable reference within quoted strings:

        ${expr}    - full indirect reference support for scalar
                     variables
        {variable} - full variable support

        For example:

          $foo[5]['bar'] = 'foobar';
          print "{$foo[5]["bar"]}";  // would print "foobar"

    * Ability to call member functions of other classes from within
      member functions or from the global scope.

      You can now, for example, override a parent function with a
      child function, and call the parent function from it.

    * Runtime information for classes (class name, parent, available
      functions, etc.).

    * Much more efficient syntax highlighter - runs much quicker,
      performs more reliably, and generates much tighter HTML.

    * A full-featured debugger has been integrated with the language
      (supports breakpoints, expression evaluation, step-in/over,
      function call backtrace, and more).

  The Zend Engine claims 100% compatability with the engine of PHP
  3.0, and is shamelessly lying about it. Here's why:

    * Static variable initializers only accept scalar values
      (in PHP 3.0 they accepted any valid expression). The impact
      should be somewhere in between void and non existent, since
      initializing a static variable with anything but a simple
      static value makes no sense at all.

    * The scope of break and continue is local to that of an
      include()'d file or an eval()'d string. The impact should
      be somewhat smaller of the one above.

    * The return statement no longer works from a require()'d file. It
      hardly worked in PHP 3.0, so the impact should be fairly small. If
      you want this functionality - use include() instead.

    * unset() is no longer a function, but a statement.

    * The following letter combination is not supported within
      encapsulated strings:  "{$".  If you have a string that includes
      this letter combination, for example, print "{$somevar"; (which
      printed the letter { and the contents of the variable $somevar in
      PHP 3.0), it will result in a parse error with the Zend Engine.
      In this case, you would have to change the code to print
      "\{$somevar"; This incompatability is due to the full variable
      reference within quoted strings feature added in the Zend
      Engine.