I see so many people finish a site and think about security afterwards (I saw your other post asking about session security).
Security items:
Your registration form has no captcha. I entered just a bad email and on the http://www.ikodeko.com/process_registration.php page got:
Notice: Undefined variable: suggest in g:\iKodeko\process_registration.php on line 57
Turn off errors from being displayed in production mode. Handle logging errors instead.
When I go to http://www.ikodeko.com/process_sign_in.php without logging in, I get:
Notice: Undefined index: username in g:\iKodeko\process_sign_in.php on line 11
Notice: Undefined index: password in g:\iKodeko\process_sign_in.php on line 12
No captcha in sign-in process either. Don't rely on register_globals being on. Turn it off and program accordingly (use $GET, $POST, etc. instead).
Delete the session cookie after user logs out and don't rely on them closing their browser.
Don't show that you use MySQL because it just will make it easier for hackers. You're already giving them a head start on how to inject SQL particular to MySQL.
If PHP security is new to you, I recommend buying this book (available for download):
http://www.phparch.com/shop_product.php?itemid=98
