I see, the id to be letters? This can be done too, but a bit more complicated. First of all, you must disable the primary key auto_increment setting for this field. Next, edit the aid data type from phpmyadmin to make it varchar(15). The last thing to do is to use the following script, which you should not have problem with:
PHP Code:
$aid = passgenerator(15,0)
Assume you do not want symbols in this field, since it is an id this should make sense. Be careful with what you do with adoptables id though, this field is predefined to be an auto-increment number and modifying this can lead to many potential problems in future.