if ($status != null) {
   if ($status == 'Outstanding') {
    $cek = $this->db->query("SELECT * FROM remark where status_case='Outstanding'");
    foreach ($cek->result() as $row) {
     $list[] = $row->remark_id ;
    }
    $this->db->where_in('remark_id',$list);

   } else if ($status == 'Receiving') {
    $cek = $this->db->query("SELECT * FROM remark where status_case='Receiving'");
    foreach ($cek->result() as $row) {
     $list[] = $row->remark_id ;
    }
    $this->db->where_in('remark_id',$list);
   }
  }

mysql where in array

-- SQL
SELECT * FROM table WHERE column IN('value1','value2','value3')

-- Javascript
$string="1,2,3,4,5";
$array=array_map('intval', explode(',', $string));
$array = implode("','",$array);
$query=mysqli_query($conn, "SELECT name FROM users WHERE id IN ('".$array."')");