What References Do

What References Do

PHP references allow you to make two variables to refer to the same content. Meaning, when you do:

it means that $a and $b point to the same content.

Note: $a and $b are completely equal here, that's not $a is pointing to $b or vice versa, that's $a and $b pointing to the same place.

Note: If array with references is copied, its values are not dereferenced. This is valid also for arrays passed by value to functions.

Note: If you assign, pass or return an undefined variable by reference, it will get created.

Example #1 Using references with undefined variables

<?php
function  foo (& $var ) { }

foo ( $a );  // $a is "created" and assigned to null

$b  = array();
foo ( $b [ 'b' ]);
var_dump ( array_key_exists ( 'b' ,  $b ));  // bool(true)

$c  = new  StdClass ;
foo ( $c -> d );
var_dump ( property_exists ( $c ,  'd' ));  // bool(true)
?>

The same syntax can be used with functions, that return references, and with new operator (in PHP 4.0.4 and later):

Since PHP 5, new return reference automatically so using =& in this context is deprecated and produces E_STRICT level message.

Note: Not using the & operator causes a copy of the object to be made. If you use $this in the class it will operate on the current instance of the class. The assignment without & will copy the instance (i.e. the object) and $this will operate on the copy, which is not always what is desired. Usually you want to have a single instance to work with, due to performance and memory consumption issues.
While you can use the @ operator to mute any errors in the constructor when using it as @new, this does not work when using the &new statement. This is a limitation of the Zend Engine and will therefore result in a parser error.

Note: If you assign a value to a variable with references in a foreach statement, the references are modified too.

Example #3 References and foreach statement

<?php
$ref  =  0 ;
$row  =&  $ref ;
foreach (array( 1 ,  2 ,  3 ) as  $row ) {
     // do something
}
echo  $ref ;  // 3 - last element of the iterated array
?>

The second thing references do is to pass variables by-reference. This is done by making a local variable in a function and a variable in the calling scope reference to the same content. Example:

will make $a to be 6. This happens because in the function foo the variable $var refers to the same content as $a. See also more detailed explanations about passing by reference.

The third thing reference can do is return by reference.