Registration Question

[deepermethod]

I am somewhat new to php/mysql so bear with me.

I have a registration form when a new user signs up. I have a field that a new member MUST fill in. The field is a "member referral" field. What I need to know is how can I check my database to verify the current members username actually exists before the new member can continue registration? If the members username is there continue. If the username is not a current member show an error.

Sorry of this seems confusing. I can post the code if needed.

Thanks.

[dagon]

select id from users where user_name=$_POST[referral]

if there's an id, they exist, else spit out error