$object1 = new SomeClass();
$object2 = $object1;

$property1 = "X";
$property2 = $property1;

Now I have objects of the same class but they are independent from each other. If I change propertyX of the $object1 propertyX of $object2 will stay unchanged. This is so called pass-by-value behavior that was default behavior in PHP 4.  We could enforce pass-by-reference bahavior with use of ampersand operator &. In PHP 5 default behavior changed a bit (btw. thanks for correcting me Phil!). Objects in PHP 5 will be passed as references (this does not apply for variables).

References in PHP are a means to access the same variable content by different names.

Assign by reference

$object1 = new SomeClass();
$object2 = & $object1;

$object1 and $object2 are completely equal here. $object1 is not pointing to $object2 or vice versa. $object1 and $object2 are pointing to the same place.

Pass by reference

class Foo {
    public $bar;
    function setBar( & $varBar) {
        $this->bar = & $bar;
    }
}

$v1 = "test";
$foo = new Foo();
$foo->setBar($v1);
$v1 = "test123";

echo $foo->bar; //echoes test123

There is no reference sign on a function call - only on function definitions. Function definitions alone are enough to correctly pass the argument by reference. As of PHP 5.3.0, you will get a warning saying that "call-time pass-by-reference" is deprecated when you use & in $foo->setBar("xxx").

Return by reference

class Foo {
    public $bar;
    function &getBar() { 
        return $this->bar;
    }
}
$v1 = "test";
$foo = new Foo();
$foo->bar = &$a;
$v2 = &$foo->getBar();
$v1 = "test123";
echo $v2; //echoes test123

Pass object by value - PHP 5

Creating a copy of an object with fully replicated properties was a default behavior back in PHP4. In PHP5 we have to enforce passing-by-value. This is done by cloning. An object copy is created by using the clone keyword (which calls the object's __clone() method if possible). An object's __clone() method cannot be called directly.

$copy_of_object = clone $object;

Further reading:

• PHP manual on references
• PHP manual on cloning

January 09, 2010     

Not so true ... ;-)

Since PHP 5, objects are always passed by reference. $o1 = new stdClass(); $o1->foo = 'bar'; $o2 = $o1; $o2->foo = 'nobar'; echo $o1->foo; // nobar That's why you have to use the 'clone' keyword to copy objects. $o1 = new stdClass(); $o1->foo = 'bar'; $o2 = clone $o1; $o2->foo = 'nobar'; echo $o1->foo; // bar
Phil | 81.62.53.33

January 09, 2010     

Thanks for corrigendum :)

Thanks Phil. I think I've corrected the mistake ;)
miha. | 193.77.156.218

January 21, 2010     

What are you supposed to use instead?

You said "As of PHP 5.3.0, you will get a warning saying that "call-time pass-by-reference" is deprecated when you use & in $foo->setBar("xxx")." but what are you supposed to use instead so you don't get a warning?
Emma | 139.166.242.91

January 21, 2010     

What are you supposed to use instead?

You said "As of PHP 5.3.0, you will get a warning saying that "call-time pass-by-reference" is deprecated when you use & in $foo->setBar("xxx")." but what are you supposed to use instead, so you don't get a warning?
Emma | 139.166.242.91