CodeIQ Q1121 Problem "Return Number"

<?php

// CodeIQ Q1121 Problem "Return Number"

$arr = array_fill(0, 10, 0);
$l = 9;

function foo($k, $f) {
    global $arr, $l;
    for ($i = 0; $i < 10; $i++) {
        $arr[$k - 1] = $i;
        $s = $f;
        for ($j = 0; $j < $k; $j++) {
            $s += $arr[$j] * $arr[$k - 1 - $j];
        }
        if ($s % 10 == $arr[$k - 1]) {
            if ($k < $l) {
                foo($k + 1, floor($s / 10));
            } elseif ($arr[$k - 1] > 0) {
                $v = '';
                for ($j = 0; $j < $k; $j++) {
                    $v .= $arr[$k - 1 - $j];
                }
                echo sprintf("%20d", $v), PHP_EOL;
                echo sprintf("%20s", bcmul($v, $v)), PHP_EOL;
            }
        }
    }
}

foo(1, 0);