On PHPMaster.com there's a new tutorial talking about randomness in PHP, what it is and some of the things it can be used for.

Use of random values is very important in the computer security field. It is crucial in computer programming for development of secure systems that are not vulnerable to malicious subversion. Cryptography relies on random value's generation and their reproducibility for unpredictable output that is core for security of any system. Random values are fundamental for secure coding practices and PHP highly makes use of them for security. You will find them used in all libraries and frameworks and almost all codes rely on them for the generation of tokens, salts and as inputs for further functions.

He talks about the important of good random numbers and some of the common uses for it including generating salts and unique identifiers. He mentions the "pseudorandomness" of PHP's generators and how they're seeded to help increase this randomness. He finishes up the tutorial with some suggestions and language features for creating "as random as possible" values like using "/dev/urandom" on Linux-based systems.