Account sign up tools

Hi! First of all, sorry for x-posting, but this question doesn't really fit in anywhere... I'm looking for a tool to enable users to sign up for an e-mail account. I'm going to open up a mail hosting service and I need some engine that can check an entered user name with the existing database so the new user can continue registration. I don't want any automatic account creation, just a check in the existing users database to see if the e-mail address is available or already taken. My e-mail system is based on Postfix, Cyrus, Web-Cyradm with a MySQL back end. I know that this should be really simple to do with a check in the MySQL data base from a php page, but I'm not at all knowledgeable in php. Anders.