I've enable register_user plugin and created register.php with below script
<?php printRegistrationForm($thanks=null); ?>
but after registration is done there is no thanks message OR error message show up, It's back to login page.
How to enable this message? How to show up below error message on register_form?

switch ($notify) {
case 'exists':
echo gettext('The user ID you chose is already in use.');
break;
case 'mismatch':
echo gettext('Your passwords did not match.');
break;
case 'incomplete':
echo gettext('You have not filled in all the fields.');
break;
case 'notverified':
echo gettext('Invalid verification link.');
break;
case 'invalidemail':
echo gettext('Enter a valid email address.');
break;
case 'invalidcaptcha':
echo gettext('The captcha you entered was not correct.');
break;

Thanks
Zham
<link removed by moderator - Please don't post links if not required in context of your post. Feel free to link your profile name. Thanks >

You need to enter the thanks message instead of the `NULL` like `<?php printRegistrationForm("Your thanks message"); ?>`as documentated.

Thank you for the prompt respond... but it wasn't help... the thanks message still not show up... I've follow your advice... but after click submit button, it's go back to login screen... no thanks message OR error message

Is it something that I do not know about PHP scripting or there is a bugs?

First off, if you are seeing an error message, then the registration was not successful so you will not get a Thank-you message. If there are no errors you will see a message regarding an e-mail that verifies the submitted email address. Registration is not complete (and acknowledged with a thank you) until this verification has happened.