而循环只设置php中输入字段中的第一个值

我想添加给定表中的用户。我正在迭代整个表，并将每个值发送到javascript文件。

<?php 
          $sql = "select * from user where user_id not in (select second_user_id from friends where first_user_id = '$user_id''n"
    . " union'n"
    . " select first_user_id from friends where second_user_id = '$user_id') limit 20";

              $result_not_friends=mysqli_query($dbc,$sql)
              or die("error in fetching");
            // print_r($row_not_friends);
           ?>
           <table class="table table-hover table-bordered">
            <h1>Users</h1>
              <tbody>
                <?php 
                  while ( $row_not_friends = mysqli_fetch_array($result_not_friends))
                  {
                    if ( $row_not_friends['user_id'] != $user_id )
                    {
                 ?>
                  <tr>
                    <td>
                      <?php echo $row_not_friends['user_name']; ?>
                    </td>
                    <!-- here I am sending request and processing it via ajax -->
                    <td><i class="fa fa-user-plus send_request"></i></td>
                    <input type="hidden" class="send_second" value="<?php echo $row_not_friends['user_id']; ?>">
                    <input type="hidden" class="send_first" value="<?php echo $user_id; ?>">
                  </tr>
                  <?php 
                    }
                    }
                   ?>
              </tbody>
          </table>

现在，我正在访问javascript文件中的每个值，如下所示：//这里有一个请求发送

$('.send_request').on('click',
    function (e) {
        e.preventDefault();
        var first = $('.send_first').val();
        var second = $('.send_second').val();
        alert('firt id is ' + first);
        alert('second id is ' + second);
        $.ajax(
        {
            url:'send_process.php',
            type:'POST',
            dataType:"json",
            data: { first: first, second: second },
            success:function(data)
            {
                if(data.send_success)
                {
                  window.location.href = "friend.php";
                }
                else
                {
                    alert("something went wrong");
                    window.location.href = "friend.php";
                }
            },
            error : function() { console.log(arguments); }
        }
            );
    }); 

但这里CCD_ 1只给出了CCD_ 2的最顶端元素值。当我回显该值时，它会给出正确的结果。请帮帮我。