Checking if a record exists with PHP and sqlite

I'm totally new to programming (normally I'm a sysadmin) but now I have to do a project where you can register and logon, with PHP and SQLite (there's no alternative).
I need to get a statement where I can do a query:

is there a user with the name from $_POST['username']

with SQLite. I just don't get it... I found some articles but none of them really helped me.