Passing by Reference

You can pass a variable by reference to a function so the function can modify the variable. The syntax is as follows:

<?php
function foo(&$var)
{
$var++;
}

$a=5;
foo($a);
// $a is 6 here
?>

Note: 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.

The following things can be passed by reference:

  • Variables, i.e. foo($a)

  • References returned from functions, i.e.:

    <?php
    function foo(&$var)
    {
    $var++;
    }
    function &    bar()
    {
    $a = 5;
    return     $a;
    }
    foo(bar());
    ?>
    See more about returning by reference.

No other expressions should be passed by reference, as the result is undefined. For example, the following examples of passing by reference are invalid:

<?php
function foo(&$var)
{
$var++;
}
function bar() // Note the missing &
{
$a = 5;
return $a;
}
foo(bar()); // Produces a notice

foo($a = 5); // Expression, not variable
foo(5); // Produces fatal error

class Foobar
{
}

foo(new Foobar()) // Produces a notice as of PHP 7.0.7
// Notice: Only variables should be passed by reference
?>