Ackersum

ackersum.php

<?php

function ackersum($a, $b) {
    if ($a == 0) {
        return $b;
    } elseif ($b == 0) {
        return $a;
    } elseif ($a == 1) {
        return $b + 1;
    }

    return ackersum(1, (ackersum($a - 1, $b) + ackersum($a, $b - 1)) / 2);
}