Some PHP libraries for generating RTF.

• rtfgen - excellent, so I've heard. But… you've got to pay for it. There's also this which may be related - there's a couple of scripts in the CVS. Otherwise The “basic” version of rtfgen, which is free, may be enough to start with.
• RTF Generator (hosted phpclasses.org - fight your way past logins and naked women to get to it ) Seems fairly sane - never tried it
• Raver RTF - no idea - looks too simple
• RTF Writer - some promising code in the CVS and looks like someone's been working on it fairly recently. No releases though

I'd love to see an RTF export (HTML→RTF or PHP→RTF) option too on all pages. That'd be great in a corporate setting.