Choosing a database system

I need help choosing a database system.

I am familiar with MySQL and SQL Server 2000, both of which I like. I like the "free" part of MySQL, but SQL Server seems to offer many more features. I am highly interested in foreign keys for my application, although not 110% necessary.

What about features like stored procedures and triggers in MySQL, do they exist? I have used them in SQL Server...

I am creating a database structure that will most likely need foreign keys to help keep data integrity. I am basically developing a discussion forum of sorts (much like this as far as structure) with many other features. A user table, posts, bridge tables linking the two, I'm projecting about 30-35 tables right now.

I am planning on using PHP. I am familiar with it, I like it, and have had little problem in using it. I have just now realized that there are SQL Server connection functions for PHP. Has anyone used these? Are they as effective as those for MySQL (which everyone seems to like using w/ PHP)?

Just looking for some suggestions on databases and such, and how I should go about all of this... Thanks

bump

I know someone has an opinion on this... Anyone?