Starting with PHP 5, the object model was rewritten to allow for better performance and more features.

This was a major change from PHP 4. PHP 5 has a full object model.

Among the features in PHP 5 are the inclusions of visibility, abstract and final classes and methods, additional magic methods, interfaces, cloning and typehinting.

<?php

class a {

    public static $foo = 'I am foo';
    public $bar = 'I am bar';
   
    public static function getFoo() { echo self::$foo;    }
    public static function setFoo() { self::$foo = 'I am a new foo'; }
    public function getBar() { echo $this->bar;    }           
}

$ob = new a();
a::getFoo();     // output: I am foo   
$ob->getFoo();    // output: I am foo
//a::getBar();     // fatal error: using $this not in object context
$ob->getBar();    // output: I am bar
                // If you keep $bar non static this will work
                // but if bar was static, then var_dump($this->bar) will output null

// unset($ob);
a::setFoo();    // The same effect as if you called $ob->setFoo(); because $foo is static
$ob = new a();     // This will have no effects on $foo
$ob->getFoo();    // output: I am a new foo

?>

PHP treats objects in the same way as references or handles, meaning that each variable contains an object reference rather than a copy of the entire object. See Objects and References

Every class definition begins with the keyword class, followed by a class name, followed by a pair of curly braces which enclose the definitions of the class's properties and methods.

The pseudo-variable $this is available when a method is called from within an object context. $this is a reference to the calling object (usually the object to which the method belongs, but possibly another object, if the method is called statically from the context of a secondary object).


<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        // Note: the next line will issue a warning if E_STRICT is enabled.
        A::foo();
    }
}

$a = new A();
$a->foo();

// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();

// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>