unable to solve a prob

Question

hi, Its a silly question but there is some issue in my calculation, as when i do

 

8535902775%10 it show -7 on my php compiler 

however i expect as it shuold be 5 i know modulo thing as currently i am not able to find my istake my program is:

 

<?php
$isbn=8535902775;
//{Write your code here
$array='0';
for($i = 1; $i<=10;$i++){
    $array += $isbn % 10;
    $isbn = $isbn/10;
    $array = $array * $i;
}
if($array/11 == 0){
    echo "Valid";
}else{
    echo "Not Valid";
}
?>