.ajax() sometimes returning errors, sometimes not - php

Question

I have the following jQuery code which, when running to the server, will sometimes return success and sometimes return error (running the alert box), depending on the user running it. It's not 100% consistent. What could be wrong? The page is loaded via an iFrame in a Facebook Tab.
The entryform.php sometimes echos and returns a message, and othertimes it returns a failure, which causes the .ajax() to trigger the error. Nothing in the server side script looks fishy though - it's set up to always return a successful message. Anything sticking out from the jQuery code?
The page returns a 200 OK status, but appears red in Firebug. What error am I missing?
jQuery:
$(document).ready(function() {
$('#entryform_submit').click(function(e) {
e.stopImmediatePropagation();
e.preventDefault();
$('#entryform_submit_loader').show();
var form_data = $('#entryform_form').serialize();
$.ajax({
url:'http://www.myurl.com/entryform.php',
type:'POST',
data:form_data,
success:function(return_data){
//do stuff
},
error:function(w,t,f){
alert('Error submitting entry. Please try again later.');
}
});
});
});

Answer 1

Got it! The key was Facebook - if a user is browsing in HTTPS, the page gets called in HTTPS - but, the .ajax() call was calling a HTTP page, which gets set as cross-domain, which is not allowed.
The key was to determine if the page is being viewed in HTTP or in HTTPS using this:
if (!empty($_SERVER['HTTPS'])) {
// https is enabled
}
And then making the appropriate AJAX call, either to the HTTP or HTTPS version.