Function for multiplication without using operators for multiplication written in PHP

As mathematic says every operations can be lowered to $x1 + $x2, but I will show multiplication which is really easy to do.
function multiplication($x1, $x2) {
    $sum = 0;
    if ($x2 > 0) {
        for ($i = 0; $i < $x1; $i++) {
            $sum += $x2;
        }
    }
	
    return $sum;
}