实现微信小程序的3rd_session

function 3rd_session($len) {

$fp = @fopen('/dev/urandom','rb');

$result = '';

if ($fp !== FALSE) {

$result .= @fread($fp, $len);

@fclose($fp);

} else {

trigger_error('Can not open /dev/urandom.');

}

// convert from binary to string

$result = base64_encode($result);

// remove none url chars

$result = strtr($result, '+/', '-_');

return substr($result, 0, $len);

}