Best way to do a weighted search over multiple fields in mysql?

Here's what i want to do:

• match a search subject against multiple fields of my table
• order the results by importance of the field and relevance of the matching (in that order)

Ex: let's assume I have a blog. Then someone searches for "php". The results would appear that way:

• first, the matches for the field 'title', ordered by relevance
• then, the matches for the field 'body', ordered by relevance too
• and so on with the specified fields...

I actually did this with a class in PHP but it uses a lot of UNIONS (a lot!) and grows with the size of the search subject. So I'm worried about performance and DOS issues. Does anybody has a clue on this?