knowing which file is including

Hi, A bit of a weird problem, am just curious if this can be done... I have a file called config.inc.php which has a require_once in it for lang_a.php and lang_b.php config.inc.php is being "included" by a.php and b.php presently config.inc.php is doing a require_once for both lang_a and lang_b, is there any way to determine which file is calling the config.inc.php (a or b) and then set the require accordingly? I dont think so but am just wondering as PHP has a crapload of solutions that I would probably never know of. Thanks, -Ryan