Register for 2 apps in one time

I would like to integrate a register for another app while registering to the forums. So you only need to sign-up once, and you can access everything. If i can put a redirect somewhere, which holds the username, password and email adress, i can make it work. (I can put a redirect at the end of the other reg page to continue the registration of the forums). My only question is: where. I tried a 100 several ways, but or it will not have the values, or it doesn't redirect. I must be missing something...The password *must* be in clear text.

Someone also suggested to add another field to the db which hold the password in cleartext. Just for the sign-up for the other app. If the other sign-up code is executed, then change the value of the field to <NULL>. It really needs to be clear text, cause it isn't asp, or another language. It is an php script which opens a socket and registers there.

Thanks for the help anyway.