Hack me! (Windows Server, IIS, PHP code)

Hi, I have a Windows Server running IIS and a PHP application. The PHP application is an API; passing certain parameters to various PHP pages returns certain data from a MySQL database. I need the server and PHP code to be secure (not military-grade, just as secure as can reasonably be implemented without a huge budget). I am looking for a freelancer to try to find security problems with either the server or with the PHP API. I will give the successful freelancer details of the server first and ask them to try to break in. I'll then give them access to the source code to see if they can find any security holes. I am therefore after someone with proven experience in computer security, and a strong knowledge of PHP. I will only consider someone experienced for this project so when submitting your application please give some details of security and PHP work you have done previously.