set differences order dependant

setdiff.php

<<?php

function setDiff($a, $b, $out=[])
{
    if ($a == [] || $b == []) {
        foreach ($a as $v) {
            $out[] = $v;
        }
        return $out;
    }

    $headA = $a[0];
    $headB = $b[0];

    if ($headA == $headB) {
        return setDiff(tail($a), tail($b), $out);
    }

    if ($headA < $headB) {
        $out[] = $headA;
        return setDiff(tail($a), $b, $out);

    }

    return setDiff($a, tail($b), $out);
}

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

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

assert([1, 3, 8] == setDiff($listA, $listB));
assert([5, 10, 12, 14] == setDiff($listB, $listA));