415. Add Strings

class Solution {

    /**
     * @param String $num1
     * @param String $num2
     * @return String
     */
    function addStrings($num1, $num2) {
        $a = str_split($num1);
        $b = str_split($num2);
        $result = '';
        $overflow = 0;
        while(!empty($a) || !empty($b)) {
            $numA = array_pop($a);
            $numB = array_pop($b);
            $sum = $numA + $numB + $overflow;
            $overflow = 0;
            if ($sum >= 10) {
                $overflow = 1;
            } 

            $result = substr($sum, -1).$result;
        }

        if ($overflow == 1) {
            return $overflow.$result;
        }

        return $result;
    }
}