Send value to a php $_GET[] from jquery tamperred navi.

Send value to a php $_GET[] from jquery tamperred navi.

Hi everyone,
this is the html navigation, generated from a php-Script in index.php

<ul id="navi" class="nav">
   <li class="level_1" id="501"><a href="index.php?id=501">root1</a></li>
   <li class="level_1" id="502"><a href="index.php?id=502">root2</a>
      <ul>
         <li class="level_2" id="503"><a href="index.php?id=503">root2.2</a>
      </ul>
   </li>
   <li class="level_1" id="504"><a href="index.php?id=504">root3</a></li>
</ul>


this is my jquery-function for it in index.php

$(document).ready(function() {
   // live
   $('#navi ul').hide();
   $('#navi li').children('a').click(function(event) {
      $(this).next().slideToggle(200);
      $('ul',$(this).parent().siblings()).slideUp(200);
      event.stopPropagation();   
      return false;
   });
// try to send the id to index.php
   $('#navi li').children('a').click(function(event) {
      var li_ID = $(this).parent().attr('id');  // get the id
      // alert(li_ID);
      $.ajax({
         type: "GET",
         async: false,
         url: "index.php",
         data: "action=" + li_ID,
         success: function(data){
            id=data;
            //alert(id);
         }
      });
   });
});


this is the part to save the id in a php-variable in index.php

$show_id = $_GET['action'];
echo $show_id;

But I get no echo from this. How can I send data from the navigation to php, all in the same index.php

what I'm doing wrong?
thanks for any advice.