I'm currently looking for a way to encrypt my users passwords, and rather than simply using crypt(), I've settled on utilizing Phpass 0.3 after reading this stackoverflow answer. However when you visit the Phpass home page, it also has the option for downloading a signature. There are two different signatures available, and it seems that you can create your own if you wanted to.
1] What is the purpose of signatures, and how do they work?
2] Are they necessary for using Phpass, or are they just an extra layer of security?