Encode long UUID to short binary string like youtube hash

uuid.php

<?php

// echo uuid_encode('cd76b808-4017-4965-b9b1-2dbcf857e405');

function uuid_encode($uuid){
    $binary = pack("h*", str_replace('-', '', $uuid));
    $binary = base64_encode($binary);
    $binary = str_replace('/', '_', $binary);
    $binary = str_replace('=', '', $binary);
    return $binary;
}

// echo uuid_decode('3GeLgARxlFabG9LLj3VOUA');

function uuid_decode($encoded){
    $encoded = str_replace('_', '/', $encoded).'==';
    $encoded = base64_decode($encoded);
    $arr     = unpack("h*", $encoded); 
    $string  = preg_replace("/([0-9a-f]{8})([0-9a-f]{4})([0-9a-f]{4})([0-9a-f]{4})([0-9a-f]{12})/", "$1-$2-$3-$4-$5", $arr[1]);
    return $string;
}

// http://www.php.net/manual/en/function.uniqid.php#94959
function gen_uuid() {
    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 )
    );
}