Ciao.
Ho questo script:
index.php
Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript" type="text/JavaScript">
/** XHConn - Simple XMLHTTP Interface - [email]bfults@gmail.com[/email] - 2005-04-08        **
 ** Code licensed under Creative Commons Attribution-ShareAlike License      **
 ** [url]http://creativecommons.org/licenses/by-sa/2.0/[/url] */
function XHConn(){
  var xmlhttp, bComplete = false;
  try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); }
  catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); }
  catch (e) { try { xmlhttp = new XMLHttpRequest(); }
  catch (e) { xmlhttp = false; }}}
  if (!xmlhttp) return null;
  this.connect = function(sURL, sMethod, sVars, fnDone){
    if (!xmlhttp) return false;
    bComplete = false;
    sMethod = sMethod.toUpperCase();
    try {
      if (sMethod == "GET"){
        xmlhttp.open(sMethod, sURL+"?"+sVars, true);
        sVars = "";
      }
      else{
        xmlhttp.open(sMethod, sURL, true);
        xmlhttp.setRequestHeader("Method", "POST "+sURL+" HTTP/1.1");
        xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
      }
      xmlhttp.onreadystatechange = function(){
        if (xmlhttp.readyState == 4 && !bComplete){
          bComplete = true;
          fnDone(xmlhttp);
        }};
      xmlhttp.send(sVars);
    }
    catch(z) { return false; }
    return true;
  };
  return this;
}

function myTest(xhr){
    alert(xhr.responseText);
}

var req= XHConn();
window.onload= function(){
    var target = document.getElementById('my-buttom');
    if(!target){return;}
    target.onclick=function(){
    var inputValue = document.getElementById('my-text').value;
    var postVar= 'cat_title='+encodeURIComponent(inputValue);
    alert(postVar);
    req.connect('test.php','POST',postVar,myTest);
    }
}
</script>
<body>
<form action="" method="post" name="frm">
<input id="my-text" name="my_test" type="text">

<input id="my-buttom" name="my_buttom" type="button" value="insert">
</form>

</body>
</html> 
test.php
Codice PHP:
<?php 
if(isset($_POST['cat_title'])){
    echo $_POST['cat_title'];
}
else{
    echo "Watch out";
}
?>
Bene provatelo se ne avete voglia
Lo script fa quello che ci si aspetta
Se apro Firebug nel reparto Net trovo le mie belle
variabili (Post cat_title mioinput Response mioinput) ma se invece dopo averlo aperto faccio partire
un'altra request trovo vuoto Post e nella Response Watch out ma nel mio debug vedo mioinput ho preso un colpo di sole o cosa