The SitePoint PHP blog has a post from author Nicola Pietroluongo talking about randomness in PHP. In the tutorial he talks about randomness, how it relates to cryptography and what's coming in PHP 7 to help.

This article analyzes problems related to random number generation used for cryptography purposes. PHP 5 does not provide an easy mechanism for generating cryptographically strong random numbers, while PHP 7 solves this by introducing a couple of CSPRNG functions.

He starts off by talking about what a CSPRNG (cryptographically secure pseudorandom number generator) is and some of the things it could be used for. He then moves on to the functionality coming in PHP 7 with the addition of the random_* functions for getting random bytes and random integer values. He talks briefly about what's going on "behind the scenes" of the generation and provides a simple code example with a randomized "dice roll" and the resulting numbers. He ends the post mentioning the random_compat library that can be installed for pre-PHP 7 applications that provides the same functionality just without those two functions defined.