You must refer to the user inputs as $_POST[aid] and $_POST[apass]. DO NOT refer to them as $aid and $apass (don't even save them as variables if possible). There are many situation where this will come back to bite you in the rear if you do. I will mention the most detrimental one which is called called sql injection. Lets assume that you are saving user data in a sql database and your form page is called rori.com. What do you think you might happen if I typed in something like rori.com?aid=drop+database in the address bar? Your code might pass $aid to the database where it will get executed. You should run some checks on $_POST[aid] and put it into something that does not resemble the variable name $aid then insert it in the database. Just google sql injection if you want a more elaborate explanation.
PS. You should thank whomever turned off global_register on the server so you can't refer to $_POST[aid] as $aid anymore. Then yell at him for ever having it turned on.