One way of looking at things is to gather the requirements for the CMS, a listing of the needs it should meet and the features it should deliver. Compare that listing to some of the Open Source CMS packages to see if there is anything out there that will do what you need.
The second thing to consider, as someone mentioned before, is time. How much time do you have to work on this? Perhaps the best of both worlds may be to use an Open Source system for the initial CMS site, and then work steadily on your own developed solution that would be more tailored to what you are expecting or wanting from it.
I just recently acquired the book Core Web Application Development with PHP and MySQL which focuses on using PHP5. I am just starting to get into the book as it arrived only late last week, but so far I feel it is a well written book and finding it quite helpful.