Function uuid

Generates a Universally Unique IDentifier, version 4.

RFC 4122 (http://www.ietf.org/rfc/rfc4122.txt) defines a special type of Globally Unique IDentifiers (GUID), as well as several methods for producing them. One such method, described in section 4.4, is based on truly random or pseudo-random number generators, and is therefore implementable in a language like PHP.

We choose to produce pseudo-random numbers with the Mersenne Twister, and to always limit single generated numbers to 16 bits (ie. the decimal value 65535). That is because, even on 32-bit systems, PHP's RAND_MAX will often be the maximum signed value, with only the equivalent of 31 significant bits. Producing two 16-bit random numbers to make up a 32-bit one is less efficient, but guarantees that all 32 bits are random.

The algorithm for version 4 UUIDs (ie. those based on random number generators) states that all 128 bits separated into the various fields (32 bits, 16 bits, 16 bits, 8 bits and 8 bits, 48 bits) should be random, except : (a) the version number should be the last 4 bits in the 3rd field, and (b) bits 6 and 7 of the 4th field should be 01. We try to conform to that definition as efficiently as possible, generating smaller values where possible, and minimizing the number of base conversions.