SitePoint.com: How to Create Your Own Random Number Generator in PHP
Computers cannot generate random numbers. A machine which works in ones and zeros is unable to magically invent its own stream of random data. However, computers can implement mathematical algorithms which produce pseudo-random numbers. They look like random numbers. They feel like random distributions. But they're fake; the same sequence of digits is generated if you run the algorithm twice.
Included in the post is code showing how to use the random functions and how to create a class (Random) that provides a few methods to help make generation easier - "seed" and "num". It first calls "seed" with a number to start the random generator off with and then "num" in a loop to pull out random values based on that.