Change content of a tab with ajax

Hi guys!

I have searched the forum, but I did not find a solution to my problem. What I am trying to do
is a list (select) where a user chooses a matchday to display. If the user chooses matchday 1
then matches from that match day show up in the tab. Here is an example of what I am trying to create. 
If you change the Spieltag value than the results change to that match day. What I got so far is this 

Controller Stats

Code:

function _fixtures(){
  @$dropdown = $_GET['matchomgang'];
 $data['matchomgang'] = $this->mdl_stats->all_games($this->uri->segment(3));
 $data['omgang'] = $this->mdl_stats->get_matchfixtures($this->uri->segment(3), $dropdown);


       $this->load->view('fixtures',$data);
}

Model Stats_model

Code:

function get_matchfixtures($compid, $matchday) {
  $table = "matcher");
   $this->db->select(" *, h.club_shortname as home, a.club_shortname as away");
           $this->db->join('clubs h', 'h.club_id = matches.matches_hteam','left');
           $this->db->join('clubs a', 'a.club_id = matches.matches_ateam','left');
           $this->db->where("comp_id = '$compid' and matchday = '$matchday' ");
       $query = $this->db->get($table);

       return $query;
   }

   function all_games($comp){
       $query = $this->db->query("SELECT distinct a.matchday FROM matches a
                                INNER JOIN
                                (
                                select comp_id,count(*) from matches
                                group by comp_id
                                having count(*) > 1
                                ) b on a.comp_id = b.comp_id
                                WHERE a.comp_id = '$comp'
                                ORDER BY a.matchday ASC");
           return $query;
   }

View

Code:

<select id="matchomgang" name="matchomgang" onchange="chooseMatchday(this.value);">
<?php foreach($matchomgang->result() as $omg){?>
<option value="<?php echo $omg->matches_matchday; ?>" >
<?php echo $omg->matches_matchday; ?>. omgang</option>
<?php } ?>
</select>

<div id="content"></div>
<ul class="matcher">
<?php foreach ($omgang->result() as $match){?>
<li><?php echo $match->home; ?> <?php echo $match->matches_hgoal; ?> : <?php echo $match->matches_agoal; ?> <?php echo $match->away; ?>
</li>
<?php } ?>

Ajax 

Code:

function chooseMatchday(str)
{
var xmlhttp;

if (str=="")
 {
 document.getElementById("content").innerHTML="";
 return;
 }
if (window.XMLHttpRequest)
 {// code for IE7+, Firefox, Chrome, Opera, Safari
 xmlhttp=new XMLHttpRequest();
 }
else
 {// code for IE6, IE5
 xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
 }
xmlhttp.onreadystatechange=function()
 {
 if (xmlhttp.readyState==4 && xmlhttp.status==200)
   {
   document.getElementById("content").innerHTML=xmlhttp.responseText;
   }
 }


xmlhttp.open("GET",'<?php echo base_url()?>/stats/_fixtures ?>/?matchomgang='+str,true);
xmlhttp.send();
}

Hopefully someone can help me find a solution to this.
Best regards.
Headpetrol