Mootools ajax request
<form name ="f1" action="">
     <select id="myr" NAME ="s1" onChange = "GetSelectedItem()">
          <OPTION VALUE = "meshed" selected >-- Please Select --</OPTION>
          <OPTION VALUE = "girls">Male seeking Female</OPTION>
          <OPTION VALUE = "mens">Female seeking Male</OPTION>
          <OPTION VALUE = "mens">Male seeking Male</OPTION>
          <OPTION VALUE = "girls">Female seeking Female</OPTION>
     </select>
</form>

$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = '';
$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die('Error connecting to mysql');
$dbname = 'ratings';
mysql_select_db($dbname) or die('Error connecting to database');
$sql = "TRUNCATE TABLE rabid_ratings";
$re = mysql_query($sql) or die(mysql_error());
echo "done";

<script type="text/javascript">
      window.addEvent('domready',function(){
          var myRequest = new Request({
               url: 'truncate.php',
               method: 'post',
               onRequest: function(){
               },
               onSuccess: function(responseText){
                    alert("done!"+ responseText);
               },
               onFailure: function(){
                    alert("failed");
               }
          });

          $('myr').addEvent('change', function(event){
               event.stop();
               myRequest.send();
          });
      });
</script>

$('mydropdown').addEvent('change', function(){
     //do your request (AJax) here
});

<select id="myr" NAME ="s1">
    <OPTION VALUE = "meshed" selected >-- Please Select --</OPTION>
    <OPTION VALUE = "girls">Male seeking Female</OPTION>
    <OPTION VALUE = "mens">Female seeking Male</OPTION>
    <OPTION VALUE = "mens">Male seeking Male</OPTION>
    <OPTION VALUE = "girls">Female seeking Female</OPTION>
</select>

<?php
  echo $_POST['s1'];
?>

window.addEvent('domready', function(){
    var myRequest = new Request({
        url: 'truncate.php',
        method: 'post',
        onRequest: function() {
        },
        onSuccess: function(responseText) {
            alert("done! " + responseText);
        },
        onFailure: function() {
            alert("failed");
        }
    });

    $('myr').addEvent('change', function(event) {
        event.stop();
        var data = this.name + '=' + this.value;  //s1=<option value>, which is the post data we are sending to the php script
        myRequest.send(data);
    });
});