Bootstrap-combobox值不在PHP中发布

I am using this Bootstrap combobox plugin (https://github.com/danielfarrell/bootstrap-combobox) which is working fine. The only problem I have is that it does not post the value of the selected item in my MySQL database. All items in my form post well except the value of this combobox which always posts as zero (0). This is my javascript:

<script type="text/javascript">
      //<![CDATA[
        $(document).ready(function(){
          $('.combobox').combobox()
        });
      //]]>
    </script>

and this is my html select:

<div class="form-group">
<label for="Site">Site:</label>
<div class="form-group">

<?php 

require "config.php";
$con = mysql_connect(DBSERVER,DBUSER,DBPASS);
mysql_select_db(DBNAME, $con);

$sql = "SELECT * from  `b17_16413362_upupa`.`site`";
$result = mysql_query($sql);

echo "<select id='Site' name='Site' class='combobox input-large form-control'>";
while ($row = mysql_fetch_array($result)) {
echo "<option value='". $row['id'] ."'>" . $row['code'] . " " . $row['site'] . "</option>";
                            }
echo "</select>";

?>
</div>
</div>

However I notice that when I change <select id="Site" name="Site" class="form-control combobox input-large"> to <select id="Site" name="Site" class="form-control"> it posts but, of course, without the auto-complete feature. So how can I integrate these two to have an autocomplete combobox that posts?

If you are using Ajax to put the data into the database, then you can get value of the input box with class combobox using simple JS as :

var boxValue = document.getElementById('site').value;

Then, you can use this JS variable to post data into your database.