用户在PHP表单中注册后我该怎么办？

I'm building my first PHP site, and I've got the registration process working even using the new PDO API, however I don't know what to do when I'm done successfully registering the user?

So, in other words, the register.php page doesn't have any markup, I just used it to POST to so I could build the record.

I'm sorry guys, I just don't yet understand how most people use PHP.

You could handle it in many different ways.

1. Have one page with markup and one page for handling post. (which is how you have it now). Your register.php page can pickup $_POST variables from your form, validate them and if ok then show quick message showing ("Well done!") echo "Well done";
2. You can also instead of showing echo "Well done", redirect to another page something like thank-you.php that will just show you have registered message. This is good because if your user tries to refresh the page it will not attempt to generate new registration. (See http://php.net/manual/en/function.header.php)
3. Handle everything in one php file. eg. If $_POST is empty then show registration form. If $_POST has items, check and validate and show message (either success or please fix following data and show form. (Example here: http://www.html-form-guide.com/php-form/php-form-tutorial.html)

Obviously there is a lot more to sending forms - validation/sanitizing being the major part of it, but since you are just learning basics I think you can forget about it for now. Just always remember to research first and try not to invent the wheel (See example http://php.net/manual/en/function.htmlspecialchars.php to help you with sanitizing)

That's up to you. Generally, you will display a registration confirmation page to let the user know that their registration worked. If you are sending an activation email, now would be a good time to do so, and to let them know as well. Also, a link to the members area from that page is helpful. You can also log the user in directly from the registration confirmation page.