Where can I start debugging php for the user/create action?

I'm trying to create a user via the REST API. I was able to do it successfully using the REST Client Firefox plugin, but when I actually try to call the URL via code, I get no response. No errors, just nothing. I'm trying to debug the PHP (I am a PHP developer), but cannot for the life of me find the point where the code starts.

I'm doing something like this:

$('#download_trial').click(function() {
            // also need to create email; can we call rest/user/register from here?
            var name = $('#mail').val();
            var data = {
                                "mail":name,
                                "account":{
                                   "name":name,
                                   "pass":generatePassword(),
                                   "mail":name
                                }
                             };
            console.log("data");
            console.log(data);
            $.ajax({
                url:'rest/user/create.json',
                type:"POST",
                data:data,
                contentType:"application/json; charset=utf-8",
                dataType:"json",
                success: function(data){
                    console.log("created user");
                    console.log(data);
                }
              });

I've tried adding a watchdog statement to the _user_resource_create function in user_resource.inc, but it never seems to be getting there. What is the initial entry point for the PHP when rest/user/create.json is called, so that I can try debugging it via my IDE debugger?