The way I've done that in the past is tediously building queries that check for the value in every text like column of every table. However, I'm recently made aware of mysql's MATCH/AGAINST. I have no experience with it, so I can't help you with the code, but check out http://dev.mysql.com/doc/mysql/en/fulltext-search.html
Apparently, if your tables are setup with fulltext indexes, then you can use MATCH/AGAINST to find text across multiple columns and tables---like you want. I hope somebody with more experience replies to this thread with a query example. Also, if you work out your solution, be sure to post a follow up here for us to learn. Thanks!