Subtract 2 number string - trừ 2 string theo kiểu số nguyên

gistfile1.txt

function super_substract($str1, $str2){
    $result = "";
    $arrs1 = str_split($str1);
    $arrs2 = str_split($str2);
    
    $r_arrs1 = array_reverse($arrs1);
    $r_arrs2 = array_reverse($arrs2);
    for($index = 0; $index < count($r_arrs1);$index++ ){
        $i1 = 0;
        if(isset($r_arrs1[$index]) ){
            $i1 = $r_arrs1[$index];    
        }
        $i2 = 0;
        if(isset($r_arrs2[$index])){
            $i2 =  $r_arrs2[$index];    
        }
        if($i1 < $i2){
            $r_arrs1[$index+1] =  $r_arrs1[$index+1] - 1;//nhớ 1
            $result[$index] =  10 + $i1 - $i2;
        }else{
             $result[$index] =  $i1 - $i2;
        }
    }
    return implode('',  array_reverse($result) );
}

$str1 = "133444433940847919255444866";
$str2 = "15433548494444426";
echo super_substract($str1, $str2);