Check and then add record

Hi PHP Gurus! I have a little problem in database. I want to add a loginID to my mysql database. As login ID is something unique, I want to first check the records in the database and see if the loginID someone is entering to database already exists. If it exisits, a message should be given else a record should be added. If ASP we do it as below. If Rs.Eof Then AddNew Else Response.Write Error End If Now this is what I want in PHP. Please help. Thanks in advance!