PHP - How to return the Server Validation Message back to Client side?

The structure of the implementation of server side validation can be adopted by the following two methods.

Method one: The PHP script does the validation is the same one that provides the user with an input form. Say form_client_server.php

Method one: The PHP script does the validation is different from the one that provides the user with an input form. Say form_client.php (i.e. print the form for the user) and form_server.php (i.e., the Action that will be triggered when the user submit the form by clicking submit button).

Here is the question:

In both cases, how the sever script can return messages (For example, error messages) t开发者_开发问答o the page that displays the form?

If possible, I would like to see a tutorial/example/book chapter that gives some detailed information for this topics.

Thank you

You insert the messages into the output HTML.