Just checking if it exists would be a select statement,
because that is the only way to get any data out of a database.
But, as someone (who's name I shamelessly forgot) pointed out in another thread, you are talking about "checking if a user already exists", which probably means that this check decides wether or not you are going to insert a record in the database.
In that case, a select is not what you want.
Why? because you can launch two copies of your script at the same time. Both will select, and both will get "user doesn't exist" then, both will decide they can insert the user, et presto, you have a double entry in your database.
Instead, force the username/date to be unique by making both columns a primary key.
Then, just try to insert a new record. If the username/date already exists, you will get a nice clean error message from the database server telling you the insert failed due to duplicates.
