Returning form variable to same page

[gyash]

I am building registeration page what i have to do is if username already present than return back to same page with all variables intact except username with message at top.."user name already exists" else to the workspace of user. How to do this...is there any redirect function available in php....also i want to know how to redirect user to their work space. can anybody give small example.
Thankx

[planetsim]

Have a Session. which collects the information in an array
Then when getting sent back. Just check if they have then output the data