A simple PHP snippet to generate UUIDv4 syntax compliant string for a given PHP request or process.

uuidv4.php

<?php

/**
 * Use PHP >= 5.3, replace any chars that don't seems relevant
 */
strtoupper(sprintf("%.8s-%.4s-4%.3s-A%.3s-%.12s",
                    md5(uniqid()),
                    md5(gethostname()),
                    md5(getmypid()),
                    md5(php_sapi_name()),
                    md5("".$_SERVER['REQUEST_TIME_FLOAT'])));

/**
 * If you want something less predictive or faster (4x)
 */
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)
);