...just to flesh out his answer, and be more explicit:
say you tell people to separate keywords with a comma. So on the page that you post to do this:
1 - explode() the keywords using your delimiter (in this case, a comma)
2 - get the # of keywords with count()
3- do a for loop and generate your WHERE clause (WHERE (field1 LIKE '%word%') OR (field2 LIKE '%word%') or (field3 LIKE '%word%)....
)