I'm just going to list some thoughts:

1. "Please Remove Install.php should die not just echo.
2. Remove XSS-able echo $_REQUEST['p'] in title
3. SQL-attack possible through $_REQUEST['p'] (Must sanitize)
4. Try and separate presentation from logic.
5. Make templates for the admin area, so you can easily change layout.

I know it's all negative here but I just flicked through the code and I'm sure over-all it's very good.

(PS:Welcome to the forums, we can never have too many Davids)

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