To answer your question over what will be best, let me first clarify what we're comparing.
PHP is a server side language that is fairly common and growing more common among web developers.
Mysql is a relational database system that is open source and free for general public use. Often used with languages such as PHP or ASP or any server side language.
WikiMedia - although I havent personally used it, is a software package. It's basically a program written in a server side language (probably PHP or ASP)
So which one you should use really depends on whether WikiMedia offers the features you need, and can do what you want to do.
There are other software packages written in PHP, ASP and MySQL that offer simliar functions. To use core PHP/MySQL means programming from scratch.
This option is naturally the most versitile and powerful, but will also be the most consuming in both time and money (at least initially)
So consider what your needs are and see if wikimedia can offer a solution. In regards to user interactivity, I'm not sure how wikimedia is set up, but they would have to give you the ability to do so, because modifying a product is often more challenging the creating it.
