Ok, I know this post was from quite a while ago but I thought I'd help out anyway.
Most Forum systems avaliable, now offer facilities whereby you can serach the data that was input into the system with relative ease. Take this vBulletin for example: You can search for most words, except maybe words lik 'it', 'was', 'and'...etc.
This is made possible by a PHP code which is executed when the post is first made. Words other than those that have been excluded are input into a database individually (along with their corresponding Titles and URLs) and can then be searched for easily by a simple SQL command : FETCH * FROM dictionary WHERE word="$search" or some such...
A good example of this simple but effective system can be found in the
PHPbb source code.
I know this thread is old, but maybe someone else might find this helpful!
~Equinox