Ways to create a unique user fingerprint in PHP

What is the best way to generate a 'fingerprint' of user unique-ness in PHP?

For example:

1. I could use a user's IP address as the 'fingerprint', however, there could be multiple other users on the same IP
2. I could use the user's IP + user agent as the 'fingerprint', however, a single user could simply swap from safari to firefox and again be seen as being unique

Ideally, the fingerprint so label the 'machine' rather than browser or 'ip' but I can't think of how this is achievable.

Open to ideas/suggestions of how you uniquely identify your users, and what advantages/disadvantages your method has.