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
function GUIDv4 ($trim = true)
{
if (function_exists('com_create_guid') === true) {
        if ($trim === true)
            return trim(com_create_guid(), '{}');
        else
            return com_create_guid();
    }
if (function_exists('openssl_random_pseudo_bytes') === true) {
$data = openssl_random_pseudo_bytes(16);
$data[6] = chr(ord($data[6]) & 0x0f | 0x40);    $data[8] = chr(ord($data[8]) & 0x3f | 0x80);    return vsprintf('%s%s-%s-%s-%s-%s%s%s', str_split(bin2hex($data), 4));
    }
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;
}
?>