non-recursive method to diff sorted sets of ints

non-recursive-diff.php

<?php

function tail(array $a)
{
    $out = [];
    unset($a[0]);
    foreach ($a as $v) {
        $out[] = $v;
    }
    return $out;
}

// Non recursive version of previous method
function setDiff($a, $b)
{
    $out = [];
    while ($a != [] && $b != []) {
        $headA = $a[0];
        $headB = $b[0];

        if ($headA == $headB) {
            $a = tail($a);
            $b = tail($b);

        } else if ($headA < $headB) {
            $out[] = $headA;
            $a = tail($a);

        } else if ($headA > $headB) {
            $b = tail($b);
        }
    }

    return array_merge($out, $a);
}

$listB = [2, 5, 9, 10, 12, 14];
$listA = [1, 2, 3, 8, 9];
/*
A = 1 2 3 8 9
B = 2 5 9 10 12 14

{A - B} = 1 3 8
{B - A} = 5 10 12 14
 */
var_dump(assert([1, 3, 8]       == setDiff($listA, $listB)));
var_dump(assert([5, 10, 12, 14] == setDiff($listB, $listA)));