hey, i'm no expert when it comes to AS, but i've actually done what you're trying to do, and instead of loading the file directly using sendAndLoad my code looks like this:
loadVariablesNum('test2.php', 0, 'POST');
//after checking that this has loaded
myXML = new XML(xmlinfo);
and in my php:
print "xmlinfo=(whatever xml you want, ensuring that you urlencode it so that the &'s do not interfere with variable definition)"
tho, since this is for an assignment, i'm not sure if you're allowed to do it like this...but my code executes perfectly for me