404 error page script

I currrently have a 404 page redirector script that I created years ago. It consists of 3 scripts. I would like to have it rewritten and cleaned up into one complete script that can then be easily installed on my other domains. The script should have a flatfile or MySQL database (currently flatfile) that contains a list of bad URLs along with a list of the correct URLs so that visitors can be redirected seemlessly from a page that has moved to its new location. This script should also have the ability to enter html for a customized 404 page if a new page location for a 404 is not in the database. If a 404 request is initiated and a redirect page is not found in the database, the script would email the information to an address so I could add the bad URL to the database with a good redirect. The above requirements should be included in an admin control panel with username/password protection. The 404 script should be developed in such a way that it can easily be installed on multiple domains. I will supply my current scripts or samples, although you are not required to use them.