com_create_guid

Here's my final version of a GUIDv4 function (based on others work here) that should work on all platforms and gracefully fallback to less cryptographically secure version if others are not supported...
<?php
/**
 * Returns a GUIDv4 string
 *
 * Uses the best cryptographically secure method 
 * for all supported pltforms with fallback to an older, 
 * less secure version.
 *
 * @param bool $trim
 * @return string
 */
function GUIDv4 ($trim = true)
{
 // Windows
 if (function_exists('com_create_guid') === true) {
 if ($trim === true)
 return trim(com_create_guid(), '{}');
 else
 return com_create_guid();
 }
 // OSX/Linux
 if (function_exists('openssl_random_pseudo_bytes') === true) {
 $data = openssl_random_pseudo_bytes(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));
 }
 // Fallback (PHP 4.2+)
 mt_srand((double)microtime() * 10000);
 $charid = strtolower(md5(uniqid(rand(), true)));
 $hyphen = chr(45); // "-"
 $lbrace = $trim ? "" : chr(123); // "{"
 $rbrace = $trim ? "" : chr(125); // "}"
 $guidv4 = $lbrace.
 substr($charid, 0, 8).$hyphen.
 substr($charid, 8, 4).$hyphen.
 substr($charid, 12, 4).$hyphen.
 substr($charid, 16, 4).$hyphen.
 substr($charid, 20, 12).
 $rbrace;
 return $guidv4;
}
?>

The phunction PHP framework (http://sourceforge.net/projects/phunction/) uses the following function to generate valid version 4 UUIDs:
<?php
function GUID()
{
 if (function_exists('com_create_guid') === true)
 {
 return trim(com_create_guid(), '{}');
 }
 return sprintf('%04X%04X-%04X-%04X-%04X-%04X%04X%04X', mt_rand(0, 65535), mt_rand(0, 65535), mt_rand(0, 65535), mt_rand(16384, 20479), mt_rand(32768, 49151), mt_rand(0, 65535), mt_rand(0, 65535), mt_rand(0, 65535));
}
?>

The output generated by the sprintf() and mt_rand() calls is identical to com_create_guid() results.

Use more cryptographically strong algorithm to generate pseudo-random bytes and format it as GUID v4 string
function guidv4()
{
 if (function_exists('com_create_guid') === true)
 return trim(com_create_guid(), '{}');
 $data = openssl_random_pseudo_bytes(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));
}

If you're going to generate random UUIDs, at least make them conform:
* The uppermost byte of the third stanza must be 4
* the uppermost byte of the fourth stanza may be any of (8 9 a b)
see also: The wikipedia page for UUIDs: https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_.28random.29 

function create_guid() { // Create GUID (Globally Unique Identifier)
 $guid = '';
 $namespace = rand(11111, 99999);
 $uid = uniqid('', true);
 $data = $namespace;
 $data .= $_SERVER['REQUEST_TIME'];
 $data .= $_SERVER['HTTP_USER_AGENT'];
 $data .= $_SERVER['REMOTE_ADDR'];
 $data .= $_SERVER['REMOTE_PORT'];
 $hash = strtoupper(hash('ripemd128', $uid . $guid . md5($data)));
 $guid = substr($hash, 0, 8) . '-' .
 substr($hash, 8, 4) . '-' .
 substr($hash, 12, 4) . '-' .
 substr($hash, 16, 4) . '-' .
 substr($hash, 20, 12);
 return $guid;
 }