function random_key($len, $readable = false, $hash = false)
{
    $key = '';

    if ($hash)
        $key = substr(sha1(uniqid(rand(), true)), 0, $len);
    else if ($readable)
    {
        $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

        for ($i = 0; $i < $len; ++$i)
            $key .= substr($chars, (mt_rand() % strlen($chars)), 1);
    }
    else
        for ($i = 0; $i < $len; ++$i)
            $key .= chr(mt_rand(33, 126));

    return $key;
}

echo random_key(12);