int mt_rand([int min], [int max]);
Many random number generators of older libcs have dubious or unknown characteristics and are slow. By default, PHP uses the libc random number generator with the rand() function. mt_rand() function is a drop-in replacement for this. It uses a random number generator with known characteristics, the Mersenne Twister, which will produce random numbers that should be suitable for cryptographic purposes and is four times faster than what the average libc provides. The Homepage of the Mersenne Twister can be found at http://www.math.keio.ac.jp/~matumoto/emt.html, and an optimized version of the MT source is available from http://www.scp.syr.edu/~marc/hawk/twister.html.
If called without the optional min,max arguments mt_rand() returns a pseudo-random value between 0 and RAND_MAX. If you want a random number between 5 and 15 (inclusive), for example, use mt_rand(5,15).
Remember to seed the random number generator before use with mt_srand().