Sending Data To Php W/ AJAX

I'm having a weird problem, not sure if I'm overlooking something or leaving something out. I'm trying to send values of a form to through AJAX to a php script then return a value. It works fine, except the author field is always blank. I've created a simplified version and it still works weird.

This is the javascript used to get the values and send them, when they come back the alert message shows Name as blank. I've looked in the PHP file to see if I made a mistake but I don't see anything.

function postComment() {
var author = document.getElementById("author").value;
var email = document.getElementById("email").value;
var comment = document.getElementById("comment").value;
var url = "includes/testComment.php";
request.open("POST", url, true);
request.onreadystatechange = showResult;
request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
request.send("author="+escape(author)+"&email="+escape(email)+"&comment="+escape(comment));}

function showResult() {
if(request.readyState == 4) {
var message = request.responseText;
alert(message);
}
}

HTML Code:
<form>Name:<br />
<input type="text" id="author" name="poster" size="25" /><br />
Email:<br />
<input type="text" id="email" name="email" size="25" /><br />
Comment:<br />
<textarea cols="36" rows="7" id="comment" name="comment"></textarea>&nbsp;
<input type="button" onclick="postComment();" name="submit" value="Comment" /></form>

PHP Code:

$author = trim($_REQUEST['author']);
$email = trim($_REQUEST['email']);
$comment = trim($_REQUEST['comment']);
$response = "Name = ".$author." email = ".$email." comment = ".$comment;
echo $response;