Description:
------------
CREATE TABLE table_name (
  id int not null primary key
);

Obviously, field `id` is an integer.
But whatever your field's type is, mysqli extension returns you string type.

P.S.: I tried sqlite3 extension, no such a problem, it returns int value.

Test script:
---------------
$db = new \mysqli('host', 'user', 'pass', 'dbname');
$res = $db->query('select * from table_name;');

$arr = $res->fetch_array(MYSQLI_ASSOC);
var_dump($arr);

$arr->free();
$db->close();

Expected result:
----------------
It should returns an array like:

array(1) {
  ["id"]=>
  int(1)
}

Actual result:
--------------
array(1) {
  ["id"]=>
  string(1) "1"
}