Ajax/jQuery/PHP in IE9 but not IE 8 or less - php

Question

My jQuery/AJAX script posts to a php file in hopes of returning XML. When I print if the data I get all the html source code returned and the xml is not properly parsed in IE 8 or less. The code works in IE 9 and all other browsers. If anyone has a suggesion to what is goin on or a solution to my problem?
jQuery:
$.post("load-content/", {type: "POST", img: placeholder, dataType: "XML", selection: $(this).html()}, function(xml) {
// format and output result
// alert(placeholder+' && '+$(this).html());
nshow = $("content", xml).text() ;
$("div p.p2").html(
$("content", xml).text()
);
alert("HERE IE 9+ "+xml.length);
});
php:
if(isset($_REQUEST["img"]) && isset($_REQUEST["selection"])) {
$xml='<ratings><content>test</content></ratings>';
echo $xml;
*FYI this code is being run in Zencart folder

Answer 1

Solved.
There were two problems here.
As the above answer suggested there was a problem with headers.
Again as the answer above suggested there was a problem with the datatype... code for older IE browsers should look something like
$.post("/load-content/", {type: "POST", img: showcase, menuv: aname}, function(response) { ...

Answer 2

Actually i think your problem is making the ajax call itself. You are using $.post but youre supplying the options hash as if it is $.ajax. The two are different...
$.post takes the url, data, callback and response type as the arguments. You are supplying a hash similar to the one you would supply to $.ajax as your second argument which is not what it wants.
If you are going to use $.post it shoudl look like this:
$.post("load-content/", {img: placeholder, selction: whatever}, function(), 'xml');
Additionally its not obivous what the contex of your call is... this may not exist unless this is inside an event handler or jQuery.each iteration so using selection: $(this).html() may not make sense.
Have you tried setting the proper header for your response and exiting immeadiately?
if(isset($_REQUEST["img"]) && isset($_REQUEST["selection"])) {
header('Content-type: application/xml');
$xml='<ratings><content>test</content></ratings>';
echo $xml;
exit(0);
}