Hey there,
I would definetly agree with laserlight, there are a lot of variables and a lot of reasons to choose what RDBMS is best. However, if you want my bias opinion, mysql and PHP work well with eachother, so a lot of developers I chat with like to work with it. And of course, with your project, you have no choice but to use some sort of database software.
It really, as laserlight mentioned, comes down to preference. I tend to find that PHP and MySQL are not only a good combo, but so many people work with it that to get help in forums become easier then lets just say, Oracle and PHP (which is what I am forced to use in some of my proejcts).
Hope it helps,
php_dev