Require to check the security system in PHP application

I have a free hosted web application built on PHP and MySQL which contains some sensitive personal data - I have coded it myself and have a decent knowledge of security but want to have someone look at it from an outsiders perspective and see what I have missed and what I need to change or improve to make it more secure. I am open to suggestions as to what should be tested based on typical scenarios.

As this is a free application I am not looking to spend a huge amount of money but I would like a good job done - please note, due to the nature of the data I will only consider those who can provide references from others whom they have done similar work for.