function diagonalDifference($arr) {
    $firstArray = [];
    $secondArray = [];
    $i = 0;
    while ($i < count($arr))
    {
        $firstArray[] = $arr[$i][$i];
        $i++;
    }
    
    $i = 0;
    $j = count($arr) - 1;
    while ($i < count($arr))
    {
        $secondArray[] = $arr[$i][$j];
        $i++;
        $j--;
    }
    
   $first = array_sum($firstArray);
   $second = array_sum($secondArray);

    return $first > $second ? $first - $second : $second - $first;
}