You can pass a variable by reference to a function.
Example:

<?php
function test (&$a)
{
    $a++;
}

$b=10;
test ($b);
// $b is 11 here
?>

Note: you can't pass NULL to a function by reference, unless it's as a default value.



You can assign default value to pass-by-reference paramter.


<?php 
function test(&$arg = 1) { 
    if ($arg == 1) { 
       echo  $arg; 
    } 
} 
test(); 
?>