Pure PHP UUID generator

demo.php

<?php

// Usage
include 'UUID.php';

// Named-based UUID.
$v3uuid = UUID::v3('1546058f-5a25-4334-85ae-e68f2a44bbaf', 'SomeRandomString');
$v5uuid = UUID::v5('1546058f-5a25-4334-85ae-e68f2a44bbaf', 'SomeRandomString');

// Pseudo-random UUID
$v4uuid = UUID::v4();

?>

UUID.php

<?php
/**
 * UUID class
 *
 * The following class generates VALID RFC 4122 COMPLIANT
 * Universally Unique IDentifiers (UUID) version 3, 4 and 5.
 *
 * UUIDs generated validates using OSSP UUID Tool, and output
 * for named-based UUIDs are exactly the same. This is a pure
 * PHP implementation.
 *
 * @author Andrew Moore
 * @link http://www.php.net/manual/en/function.uniqid.php#94959
 */
class UUID
{
	/**
	 * Generate v3 UUID
	 *
	 * Version 3 UUIDs are named based. They require a namespace (another 
	 * valid UUID) and a value (the name). Given the same namespace and 
	 * name, the output is always the same.
	 * 
	 * @param	uuid	$namespace
	 * @param	string	$name
	 */
	public static function v3($namespace, $name)
	{
		if(!self::is_valid($namespace)) return false;

		// Get hexadecimal components of namespace
		$nhex = str_replace(array('-','{','}'), '', $namespace);

		// Binary Value
		$nstr = '';

		// Convert Namespace UUID to bits
		for($i = 0; $i < strlen($nhex); $i+=2) 
		{
			$nstr .= chr(hexdec($nhex[$i].$nhex[$i+1]));
		}

		// Calculate hash value
		$hash = md5($nstr . $name);

		return sprintf('%08s-%04s-%04x-%04x-%12s',

		// 32 bits for "time_low"
		substr($hash, 0, 8),

		// 16 bits for "time_mid"
		substr($hash, 8, 4),

		// 16 bits for "time_hi_and_version",
		// four most significant bits holds version number 3
		(hexdec(substr($hash, 12, 4)) & 0x0fff) | 0x3000,

		// 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
		(hexdec(substr($hash, 16, 4)) & 0x3fff) | 0x8000,

		// 48 bits for "node"
		substr($hash, 20, 12)
		);
	}

	/**
	 * 
	 * Generate v4 UUID
	 * 
	 * Version 4 UUIDs are pseudo-random.
	 */
	public static function v4() 
	{
		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)
		);
	}

	/**
	 * Generate v5 UUID
	 * 
	 * Version 5 UUIDs are named based. They require a namespace (another 
	 * valid UUID) and a value (the name). Given the same namespace and 
	 * name, the output is always the same.
	 * 
	 * @param	uuid	$namespace
	 * @param	string	$name
	 */
	public static function v5($namespace, $name) 
	{
		if(!self::is_valid($namespace)) return false;

		// Get hexadecimal components of namespace
		$nhex = str_replace(array('-','{','}'), '', $namespace);

		// Binary Value
		$nstr = '';

		// Convert Namespace UUID to bits
		for($i = 0; $i < strlen($nhex); $i+=2) 
		{
			$nstr .= chr(hexdec($nhex[$i].$nhex[$i+1]));
		}

		// Calculate hash value
		$hash = sha1($nstr . $name);

		return sprintf('%08s-%04s-%04x-%04x-%12s',

		// 32 bits for "time_low"
		substr($hash, 0, 8),

		// 16 bits for "time_mid"
		substr($hash, 8, 4),

		// 16 bits for "time_hi_and_version",
		// four most significant bits holds version number 5
		(hexdec(substr($hash, 12, 4)) & 0x0fff) | 0x5000,

		// 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
		(hexdec(substr($hash, 16, 4)) & 0x3fff) | 0x8000,

		// 48 bits for "node"
		substr($hash, 20, 12)
		);
	}

	public static function is_valid($uuid) {
		return preg_match('/^\{?[0-9a-f]{8}\-?[0-9a-f]{4}\-?[0-9a-f]{4}\-?'.
                      '[0-9a-f]{4}\-?[0-9a-f]{12}\}?$/i', $uuid) === 1;
	}
}
?>

Added time-based v1 uuid generation.

Well, you can consider using this library, easy to use!