I have a lynda.com account and I strongly recommend purchasing a one month pass ($25) and working through the Kevin Skoglund video series. I would be more than willing to help you if you get stuck in the video series, as I got stuck in three or four spots. Going through that series gave me a solid understanding of how PHP and MySQL work together.
The fact of the matter is these products enable people like me (part-time designer) to do back-end work without having to pay an expert $100-$200 an hour. Add to that the help of Ray and others on this forum and I have successfully completed my first e-commerce site in less than two weeks.
I think your question is a good one (inventory control) and I will look into as well, because I know that I may need that down the line too. I say keep the thread going and allow others to chime in so that we can work together to find a solution.
For MySQL questions: www.mysql.com