It's fairly common practice these days for web sites that require simple user authentication to store the users passwords in a hashed form rather than plain text.

Added register_user() as suggested, fixed a small syntax error -- missing { in show_last_vist() -- and added the SQL needed to create the table in phpMyAdmin or at the MySQL command prompt.

Just a simple proof-of-concept script showing the basic login / logout process. Added the register_user() function and fixed a small syntax error.