Multiply Big Number v4

<?php
/**
 * Created by PhpStorm.
 * User: RazielVonChronos
 * Date: 25-Nov-18
 * Time: 13:34 PM
 */

$number = strval(readline());
$n = intval(readline());
// we are doing the match backwards so we need to take the index of the value and multiply it for each step.
$index = 1;
// put them in array so we can observe each record's behaviour
$array = [];

for ($i = (strlen($number) - 1); $i >= 0; $i--) {
    $array[] = sprintf('%.0f', ($number[$i] * $n) * $index);
    // $i's position defines how big the number(via $index) example 1, 10,100,100 and etc..
    $index = $index * 10;
};

//var_dump($array);
// calling sprintf in order to print the big number normally.
// using doubleval for more memory (big numbers sux)
echo sprintf("%.0f", array_sum(array_map('doubleval', $array)));

/*
при вход:
923847238931983192462832102
4
изхода се очаква да е
3695388955727932769851328408
но ми връща
3695388955727932301651214336
*/