Solution.php

<?php
// https://leetcode.com/problems/add-strings/
class Solution
{

    /**
     * @param String $num1
     * @param String $num2
     * @return String
     */
    function addStrings($num1, $num2)
    {
        $result = "";
        $num1Len = strlen($num1);
        $num2Len = strlen($num2);
        $length = $num1Len > $num2Len ? $num1Len : $num2Len;
        $num1 = strrev($num1);
        $num2 = strrev($num2);
        $addDigit = 0;
        for ($i = 0; $i < $length; $i++) {
            $sum = (int)$num1[$i] + (int)$num2[$i] + $addDigit;
            if ($sum >= 10) {
                $addDigit = floor($sum / 10);
            } else {
                $addDigit = 0;
            }
            $result .= $sum % 10;
        }
        if ($addDigit != 0) {
            $result .= $addDigit;
        }
        $result = strrev($result);
        return $result;
    }
}

$num1 = "9333852702227987";
$num2 = "85731737104263";

$num1 = "3876620623801494171";
$num2 = "6529364523802684779";
$obj = new Solution();
$res = $obj->addStrings($num1, $num2);
var_dump($res);