You may want to take a look a Request Tracker. I believe that it already does what you are after. Here is an excerpt from the the Features section of the website.

Features

RT is a robust ticketing system designed for the small-to-medium sized enterprise. It can be used for customer support, bugtracking, trouble ticketing, or any other purpose for which a group of folks need to keep track of a list of tasks. It features a number of technologies designed to make it easy to keep track of a large number of tasks, issues and projects:

• Web, email and commandline interfaces. The web interface is designed to work well with all popular browsers, including console-based browsers.
• Web interface for each end-user to quickly check on the status of his open tickets.
• Data is stored in an SQL database which you can audit and report on using standard tools.
• Granular access control system which allows users to do their work but also protects the system.
• Flexible 'keywords' system which allows a local site to keep track of ticket metadata in the most sensible way.
• Extensible linking system which allows you to tie tickets to each other and to external databases. (RT supports dependencies, parent-child relationships and generic relationships)
• Clean object oriented design makes it a breeze to build new tools which work with RT's tickets, queues, users and links.
• Open 'Scrips' system allows sites to drop in custom business logic which persists transparently across system upgrades.
• Doesn't cost anything to set up and use. (Available under the terms of the GNU GPL)

Being someone who has tried to build a support desk, I would strongly recommend using free stuff that already exists. If you still want to do it yourself, then have a look at what other people have done.

Kristofer A. Hoch