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: prop = 'Hello'; $other = new Foo; $other->prop = 'Bye'; $object->replace($other); print $object->prop; // still shows 'Hello' ?> * 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: 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: aPrivateMethod(); } } class Bar extends Foo { public function aPublicMethod() { echo "Bar::aPublicMethod() called.\n"; $this->aProtectedMethod(); } } $o = new Bar; $o->aPublicMethod(); ?> Old code that has no user-defined classes or functions named 'public', 'protected' or 'private' should run without modifications. * Abstract Classes and Methods. The Zend Engine 2.0 introduces abstract classes and methods. An abstract method only declares the method's signature and does not provide an implementation. A class that contains abstract methods needs to be declared abstract. Example: test(); ?> Classes that do not have abstract methods can be declared abstract to prevent them from being instantiated. Old code that has no user-defined classes or functions named 'abstract' should run without modifications. * Interfaces. The Zend Engine 2.0 introduces interfaces. A class may implement an arbitrary list of interfaces. Example: Old code that has no user-defined classes or functions named 'interface' or 'implements' should run without modifications. * Class Type Hints. While remaining loosely typed the Zend Engine 2.0 introduces the ability to use class type hints to declare the expected class of objects that are passed as parameters to a method. Example: a($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 members and methods. Those cannot be overridden by sub-classes. Example: 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: Properties cannot be final. 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 calling the object's __clone() method. Example: __clone(); ?> 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 will supply a function that imports all of the properties from the source object, so that they can start with a by-value replica of the source object, and only override properties that need to be changed. [The function hasn't been implemented yet] Example: id = self::$id++; } function __clone() { $this->name = $that->name; $this->address = 'New York'; $this->id = self::$id++; } } $obj = new MyCloneable(); $obj->name = 'Hello'; $obj->address = 'Tel-Aviv'; print $obj->id . "\n"; $obj = $obj->__clone(); print $obj->id . "\n"; print $obj->name . "\n"; print $obj->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: 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: 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: 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 compare 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: 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 proerty and hence there getter methods. As the code shows it is possible to read any available information by using the getter methods. But since some og 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: 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. * Dereferencing objects returned from functions. Example: draw(); ShapeFactoryMethod('Square')->draw(); ?> * Member variables of classes can now be initialized. Example: 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: The pseudo variable $this is not available inside a method that has been declared static. * instanceof. New support for an instanceof operator which checks if an object is of a certain class or interface type. Example: * Static function variables. Statics are now treated at compile-time which allows developers to assign variables to statics by reference. This change also greatly improves their performance but means that indirect references to statics will not work anymore. * Parameters that are passed by reference to a function may now have default values. Example: * __autoload(). The __autoload() interceptor function will be automatically called when an undeclared class is to be instantiated. The name of that class will be passed to the __autoload() interceptor function as its only argument. __autoload() must succeed in loading the class. If it doesn't then an E_ERROR is emitted. Example: * Method calls and property accesses can be overloaded by class methods __call(), __get() and __set(). __get() and __set() Example: 1, 'b' => 2, 'c' => 3); function __get($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: 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: $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: obj = $obj; } function rewind() { $this->num = 0; } function hasMore() { 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->hasMore(); $it->next) { $key = $it->current(); $val = $it->key(); echo "$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. * __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: * __toString() The magic method __toString() allows to overload the object to string conversion. This conversion is only done automatically for printing functions (echo, *printf) 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 maybe converted to a string in any place but without calling __toString(). Example: * Reflection API PHP5 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 separatley: http://sitten-polizei.de/php/reflection_api/docs/language.reflection.html Example: * 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.