Can't Create DOM Document

I'm trying to create a php form which is auto completed after onchange
action of a drop down.

I've created a js file and linked it to a PHP file, which is linked
with a XML file.

Inside the PHP - after i tried to print the response and got a blank
one:

var response = xmlHttp.responseText;
alert(response);

I've tried to print after creating the DomDocument, but it probably
doesn't work, because it doesn't print anything:

<?
$q=$_GET["q"];
$xmlDoc=new DomDocument();
print $q;
.....
.....
?>

If i put the print $q before the Dom creation: $xmlDoc=new
DomDocument(); it does print.

How can i create the DOM without an error? where is my mistake?