Passing arguments to a function by reference
<?php
function inc(& $b) {
    $b++;
}
$a = 1;
inc($a);
echo $a;
?>
Output
2
A function may return a reference to data as opposed to a copy 

<?php
function & get_data() {
    $data = "Hello World";
    return $data;
}
$foo = & get_data();
?>