<?php

$arr = array(1, 2, 3, 4, 5);

function diff (&$arr) {
  $arr = array_diff($arr, array(2, 4));
}

print_r($arr);
diff($arr);
print_r($arr);

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
)
Array
(
    [0] => 1
    [2] => 3
    [4] => 5
)