AJAX与PHP问题

I've been trying to use AJAX within my web application to speed things up. I have two files, readdata.php, which contains the html and ajax request, and data.php which echos out the text information that I need to be asynchronously updated.

Here are the two files:

readdata.php
<!DOCTYPE html>
<html>
<head>
    <title>Read Data</title>

    <?php
//these two posts are required to access the database and are passed through a submit button from index.php
        $rName=$_POST["registration"];  
        $rowId=$_POST["rowid"];
        ?>
<script>
function showUser(str) {    //mostly copied from online ajax tutorial
  if (str=="") {
    document.getElementById("txtHint").innerHTML==xmlhttp.responseText;
    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("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
  xmlhttp.open("POST","data.php",true);
  xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
}
</script>
</head>
<body>

<br>
<div id="txtHint"><b>data.php text will be listed here.</b></div>

</body>
</html>

and the other:

data.php
<?php
    echo "Hello World";  //this is the text that I want to be displayed and asynchronously updated.

?>

</body>

</html>

When I go to index.php and click on the button linking to readdata.php, only the text "data.php text will be listed here." is displayed. Any help would be greatly appreciated.

Allright,

I will try to make it as easy as possible.

I always recommend using jQuery (If so, include the latest API or place it on your host)

$.ajax({
  type: "POST", //Or even get
  url: "read.php",
  data: { 
    //Theese are the variables and their relative values
      name: "Brad", 
      surname: "Pitt" 
  }
})
  .done(function( msg ) {
    alert( "Result: " + msg );

});

Whatever will be echoed from read.php, will be shown on the alert. You can call this script by placing it inside a function() {} like:

function syncWithAjax() {
$.ajax({
  type: "POST",
  url: "read.php",
  data: { 
      name: "Brad", 
      surname: "Pitt" 
  }
})
  .done(function( msg ) {
    alert( "Result: " + msg );

});
}