Sum tow very very big number in php(ex:999999999999999999999999999999999999+999999999999999999999999999999999999)

big_sum.php

<?php
/**
 * Sum tow very big number
 * @param $n1
 * @param $n2
 * @return string
 */
function big_sum($n1,$n2)
{
    if(strlen($n1) > strlen($n2)){
        //small number
        $m1 = $n2;
        //big number
        $m2 = $n1;
    }else{
        //small number
        $m1 = $n1;
        //big number
        $m2 = $n2;
    }
    $m1     = str_repeat(0,(strlen($m2) - strlen($m1))).$m1;
    $len    = strlen($m1)-1;
    $re     = "";
    $plus   = 0;
    for($i = $len;$i >= 0;$i--){
        $n1 = (int)$m1[$i];
        $n2 = (int)$m2[$i];
        $s  = $n1 + $n2 + $plus;
        if($s >= 10){
            $re     = substr($s,strlen($s)-1,strlen($s)).$re;
            $plus   = (int)substr($s,0,strlen($s)-1);
        }else{
            $re     = $s.$re;
            $plus   = 0;
        }
    }
    return (($plus == 0) ? "" : $plus).$re;
}
/**
 * @param $n1
 * @param $n2
 * @return mixed
 */
function big_multiplication($n1,$n2)
{
    if(strlen($n1) > strlen($n2)){
        //small number
        $m1 = $n2;
        //big number
        $m2 = $n1;
    }else{
        //small number
        $m1 = $n1;
        //big number
        $m2 = $n2;
    }
    $re = $m2;
    for($i = 1;$i < $m1;$i++){
        $re = big_sum($m2,$re);
    }
    return $re;
}