สอบถามการใช้ explode แล้วขึ้นเป็น ArrayArrayArrayArrayArrayArrayArray

<?php
  $sel = $db->prepare('SELECT * FROM tbl_borrow_logs');
        $sel->execute();
      while ($result = $sel->fetch(PDO::FETCH_ASSOC))
              { 
                //Explodes checkbox values
          $field = explode("-", $result['nb_selected']);
         echo "<pre>";
         print_r($field);
            // for($i = 0; $i < count($field); $i++){
            //   if ($field[$i] == $result['list']) {
            //     echo $result['list'];
            //   }
            // }
        }
?>

<?php
  $sel = $db->prepare('SELECT * FROM tbl_borrow_logs INNER JOIN tbl_machine ON tbl_borrow_logs.nb_selected = tbl_machine.id');
        $sel->execute();

      while ($result = $sel->fetch(PDO::FETCH_ASSOC))
              { 
                //Explodes checkbox values
               $field = explode("-", $result['nb_selected']);
                foreach($field as $r) {
                  if ($r == $result['id']) {
                    echo $result['list'];
                    // echo "<pre>";
                    // print_r($rr);
                  }
            }
         //print_r($field);
        }
?>