I really like the way Bitt's ROT13 spoiler works, so I am going to implement it on the board. One piece I need though is PHP 4 code to do the following:

Take the string $Body, and ROT13 text between [rot13] this text [/rot13].

I'm horrible with string manipulation code, so I'm hoping someone here well versed in PHP can make the one line needed to do this.