PHP function to generate v4 UUID

Question

So I've been doing some digging around and I've been trying to piece together a function that generates a valid v4 UUID in PHP. This is the closest I've been able to come. My knowledge in hex, decimal, binary, PHP's bitwise operators and the like is nearly non existant. This function generates a valid v4 UUID up until one area. A v4 UUID should be in the form of:

xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx

where y is 8, 9, A, or B. This is where the functions fails as it doesn't adhere to that.

I was hoping someone with more knowledge than me in this area could lend me a hand and help me fix this function so it does adhere to that rule.

The function is as follows:

<?php

function gen_uuid() {
 $uuid = array(
  'time_low'  => 0,
  'time_mid'  => 0,
  'time_hi'  => 0,
  'clock_seq_hi' => 0,
  'clock_seq_low' => 0,
  'node'   => array()
 );

 $uuid['time_low'] = mt_rand(0, 0xffff) + (mt_rand(0, 0xffff) << 16);
 $uuid['time_mid'] = mt_rand(0, 0xffff);
 $uuid['time_hi'] = (4 << 12) | (mt_rand(0, 0x1000));
 $uuid['clock_seq_hi'] = (1 << 7) | (mt_rand(0, 128));
 $uuid['clock_seq_low'] = mt_rand(0, 255);

 for ($i = 0; $i < 6; $i++) {
  $uuid['node'][$i] = mt_rand(0, 255);
 }

 $uuid = sprintf('%08x-%04x-%04x-%02x%02x-%02x%02x%02x%02x%02x%02x',
  $uuid['time_low'],
  $uuid['time_mid'],
  $uuid['time_hi'],
  $uuid['clock_seq_hi'],
  $uuid['clock_seq_low'],
  $uuid['node'][0],
  $uuid['node'][1],
  $uuid['node'][2],
  $uuid['node'][3],
  $uuid['node'][4],
  $uuid['node'][5]
 );

 return $uuid;
}

?>

Thanks to anyone that can help me out.

Answer 1

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 )
    );
}