Hi Helena,
You could use a table in your database for user authentication. This table should contain an id number, username and password. You should include a simple header file into your PHP pages. So when a page is requested the user will get an Username and Password window which is the same like the one you see when you're entering a password protected directory. Then the script will check if the user with the password is in the database. If not, the script prints an error message. If the user is in the database he or she will see the requested page without anything like an id, password or username variable. (So: not index.php?user=helena&password=bla, but just index.php)
If you give me your e-mailadres I will e-mail the script to you. (I'm from The Netherlands, so I can speak Dutch if you want).
