Retreiving a session varible and querying db if found redirect user.

[underground]

I have a question more so I need a example of how to achieve this type of php function. I don't have any script I been playing around with the idea in my head and need some expert opinion as well recommendation.

In this senerio I am using session's to control access to specific pages.
The main page is a control panal were the user can create account are a profile.
kinda like myspace.

I only want to allow them to create one account so the user name is passed
to the member and picture tables. I want to use this as the primary key to keep the user from creating more that one account with the same name..

Now my question is how can I prevent the user from accessing the account set page but rather display existing account info and allow them to update or make changes to the member and picture tables...
=============== =============== =============== =============
scenerio:
retreive session name==>querydb= =>display result based on query==>
results: user name not found ===show profile set up page=>
results:user name found==display edit page=>