Word search in php & MySQL

For a year now I've been the webmaster of a phpBB forum, and regularly I'm surprised by how useless
the forum's "search" functionality is. The database is not big (as of now there are some 20
active users, and around 5000 messages) but the "search" still makes crass mistakes like being unable to find a
word present in some topic's title. Usually, I'm better off googling the keyword(s) + my domain name.

The rest of phpBB seems pretty cool and professional to me.
So every now and then I wonder if I might implement a better search program on my own.
Is there any "theory", any "best practices" of word search in forums with MySQL databases ?