Passing by Reference

You can pass variable to function by reference, so that function could modify its arguments. The syntax is as follows:

function foo (&$var)

foo ($a);
// $a is 6 here

Note that there's no reference sign on function call - only on function definition. Function definition alone is enough to correctly pass the argument by reference.

Following things can be passed by reference:

Any other expression should not be passed by reference, as the result is undefined. For example, the following examples of passing by reference are invalid:

function bar() // Note the missing &
    $a = 5;
    return $a;

foo($a = 5) // Expression, not variable
foo(5) // Constant, not variable

These requirements are for PHP 4.0.4 and later.