uuid 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 )

);