PHP function to generate v4 UUID

Instead of breaking it down into individual fields, it's easier to generate a random block of data and change the individual byte positions. You should also use a better random number generator than mt_rand().

According to RFC 4122 - Section 4.4, you need to change these fields:

1. time_hi_and_version (bits 4-7 of 7th octet),
2. clock_seq_hi_and_reserved (bit 6 & 7 of 9th octet)

All of the other 122 bits should be sufficiently random.

The following approach generates 128 bits of random data using openssl_random_pseudo_bytes(), makes the permutations on the octets and then uses bin2hex() and vsprintf() to do the final formatting.

function guidv4($data){    assert(strlen($data) == 16);    $data[6] = chr(ord($data[6]) & 0x0f | 0x40); // set version to 0100    $data[8] = chr(ord($data[8]) & 0x3f | 0x80); // set bits 6-7 to 10    return vsprintf('%s%s-%s-%s-%s-%s%s%s', str_split(bin2hex($data), 4));}echo guidv4(openssl_random_pseudo_bytes(16));

With PHP 7, generating random byte sequences is even simpler using random_bytes():

function guidv4($data = null){    $data = $data ?? random_bytes(16);    // ...}

Taken from this comment on the PHP manual, you could use this:

function gen_uuid() {    return sprintf( '%04x%04x-%04x-%04x-%04x-%04x%04x%04x',        // 32 bits for "time_low"        mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff ),        // 16 bits for "time_mid"        mt_rand( 0, 0xffff ),        // 16 bits for "time_hi_and_version",        // four most significant bits holds version number 4        mt_rand( 0, 0x0fff ) | 0x4000,        // 16 bits, 8 bits for "clk_seq_hi_res",        // 8 bits for "clk_seq_low",        // two most significant bits holds zero and one for variant DCE1.1        mt_rand( 0, 0x3fff ) | 0x8000,        // 48 bits for "node"        mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff )    );}

Anyone using composer dependencies, you might want to consider this library: https://github.com/ramsey/uuid

It doesn't get any easier than this:

Uuid::uuid4();