KohanaPHP » Kohana v3.x

Would php's uniqid() suffice?

It is not the same really. Uniqid() is not RFC4122 compliant, it is just a "unique" string based on current time.
If you use it multiple times, you will get very similar output strings and everyone who is familiar with uniqid() will recognize the source algorithm. Further more, GUID is used extensively in RSS, syndication protocols and open protocols of all kind. It is also useful to generate token keys.
http://en.wikipedia.org/wiki/UUID

Though, there are multiples implementations of UUID (same as GUID) in the comments of PHP.net page
http://php.net/manual/en/function.uniqid.php

The implementation above is generating Version 4 UUID (random numbers), but implementing version 5 (based on namespace and name hashed with sha1) could be useful as well. There are also some v4 cryptographically secure implementations, this one in particular being interesting :
http://www.php.net/manual/en/function.uniqid.php#88400

I've added a helper class to a custom Kohana module for this one. I'm using UUID all over the place in my database. It's using the PECL UUID module, when available and it will fall back to the cryptographically secure implementation, mentioned by Cédric. The PECL UUID is a lot faster, so you will get a major advantage when installed.

See here for the source: http://pastie.org/915501

There's quite a decent PHP implementation here: http://www.shapeshifter.se/2008/09/29/uuid-generator-for-php/