Codeigniter查询（更新）数据库onclick

i am building a notification system for my project,i am more of a php developer what i want to accomplish is that when a user clicks on a notification link eg [3] means there are 3 unread notifications,what i want is that when a user clicks and see these notifications to update my notification table and set seen field to 1 so as a notification link will be [0] and i know this can be achieved through ajax request or any other method if anyone can help will appreciate a lot coz m finalizing my project and run out of time :-) here z my code

          <?php if (isset($this->session->userdata['messagecount'])): ?>
              <?php if (empty($this->session->userdata['messagecount'])){echo "<button class='btn'></button>";}else{

                  $count=$this->session->userdata['messagecount'];

     echo "<button class='btn btn-info dropdown-toggle notify-btn' data-toggle='dropdown' onclick='updateNotifications()'href='#'>".$count."</button>"; 
                  }?>


     <?php endif; ?>

        <ul class="dropdown-menu">
         <div class="notify-wrapper">
          <div class="scroll">
            <div class="scroll-body">
               <ul class="notification-list">
            <?php if (isset($this->session->userdata['notification'])): ?>
              <?php if (empty($this->session->userdata['notification'])){echo "No Notifications";}else{

                  $notification=$this->session->userdata['notification'];

                  echo $notification; 
                  }?>


     <?php endif; ?>
            </ul>
            </div>
            </div>
            </div>
        </ul>


   <?php }else{?>



   <?php }?>
 </li>

Try this dirty code and change as per your requirements:

<div id="notification_counter"><?php echo $note; ?></div>
<div id="notification_container">
         <ul>
                <li class= "notification_heading" onClick="read_notification_url">one</li>
                <li class= "notification_heading" onClick="read_notification_url">two</li>
                <li class= "notification_heading" onClick="read_notification_url">three</li>                
         </ul>
</div>

<script>
$(function() {
        $(".notification_heading").click( function(){
                 var rquest = $.ajax({
                            type: "POST",
                            url: "url_to_modify_counter",
                          });
                          rquest.done(function( msg ) {                              
                                $('#notification_counter').val($('#notification_counter').val()-1);
                              }
              });
          });
</script>

be sure to make it safe and data type conversion and all security checks