Why is this Javascript not triggering an Ajax event?

I have a forum of sorts, and I want to automatically refresh the posts every so often. I am using Ajax when the page loads to start and later I will implement the auto refresh. The problem I am having is (I believe) inducing a Ajax request. I have an event handler for the Ajax request to take place when I click my header (for purposes of debugging). I won't post all of my code, just the most relevant sections here, but if you want you think there is probably a problem in my code elsewhere, feel free to check out www.ethoma.com/testhome.p开发者_如何学JAVAhp which sends a request to www.ethoma.com/getposts.php.

Ajax requesting function:

function getPosts(category, page, sort)
{
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("postcontainer").innerHTML=xmlhttp.responseText;
        }
}
var queryString = "?category=" + category + "&page=" + page + "&sort=" + sort;
xmlhttp.open("GET","getuser.php" + queryString,true);
xmlhttp.send();
}

My PHP page should return the correct html code through an echo call. On a side note, it is okay to embed html tags within the code I return through my PHP page (novice question)?

If you think the problem is not in this code snippet, again feel free to browse those two pages. Thanks to everyone who views/answers this question -- everyone here is very helpful.

I've looked at the live version of your site and ran it with a breakpoint in onreadystatechange. The reason you don't see anything is that getuser.php 404's.

In particular the called URL is http://www.ethoma.com/getuser.php?category=[object%20HTMLAllCollection]&page=1&sort= (I'm quite sure the category is a bug) and yields 404, onreadystatechange is then called with readyState=2 and status=404