I read about Frapi in PHP Architect (May 2011), and spent a couple of hours trying it out. It is quite interesting, but I don't think I will be using it. It is a full web-interface for making the API. This is what makes it cool, but also its biggest disadvantage. There is a lot of code involved, meaning there is a lot to learn if you need to change it and lots of places for bugs and security exploits to crop up.
comes with a documentation generator, which could be really useful. This
feature is still incomplete (for instance there are no links to it yet, see here,
PDF generation didn't work properly), but it looks okay.
is one specific limitation: I could not create an action with an
optional parameter, at least not using the router. E.g. if my action is
called "ddd", then I can call /ddd (no parameter). But I cannot call
"/ddd/77". (I can give an optional parameter with /ddd?id=77). Or I can
define a router as "/ddd/:id" so that I can call "/ddd/77". But in that
case id is now required and I cannot use "/ddd".
Another disadvantage is no built-in support for oAuth. (I did find an oauth extension for Frapi but did not try it as the integration seems quite rough still.)
Incidentally if you were looking for a
full application built on Zend Framework this may make the perfect study
case. Apparently only the admin interface uses ZF, and the actual web
services do not; but as far as I can tell they are closely tied and you
need to keep both together even on your production servers.
Overall, because making a web service in PHP is not that hard, the advantages are
slight and not enough to outweigh the disadvantages (large codebase, inflexible structure, etc.).