Edit: just looked at an old project where I used ajax to call php files, and I was just specifying the whole url. So I would expect that "http://www.bbb.com/a.php" would work. (The file I was calling was up twp levels, then down another directory.)
Yea, like AllanR says, just call the base URL exactly where your application is: "http://www.bbb.com/a.php"