Complex SQL search query with relevance ordering

Hello everybody. I'm having difficulty generating an sql query, that will give me a desired result, so I'm asking for help. I have the following need: A search term is submitted, that can contains N amount of words. I need a query that will search for a combination of *at least 2 occurrences anywhere in the value of the fields* to *results containing all of the words* (expecting the word delimiter to be a "space"), and this search will look in 3 db fields (title, keywords, notes) The result I need ordered by relevance, top results to be any matches with *all word occurrences* and lower matches to follow. Each field has the same priority so if a search has 4 words, 4 matches in either title, or keyword have the same weight. It's not possible to predict how many words there may be in the string. This will be run through php, so anything that cannot be done with a query alone, can be done there, but i'm aiming for a "as much as possible" mysql query.