On PHPMaster.com today there's a new tutorial showing how to do some form validation using some basic PHP (no external libraries or tools here). This is a beginner level tutorial to help you get familiar with the concepts behind doing validation (and

In this article you’ll construct and validate a simple form using HTML and PHP. The form is created using HTML and validation and processing of the form’s contents is done with PHP. The goal is to teach you some basic HTML form elements and how their data is accessible to you in your PHP scripts.

They start with the form itself, a basic setup with various kinds of fields - text, select, radio and a checkbox. Both the HTML markup and the PHP to do the validation is included. They check for things like "not empty", "must select one" and optional fields. Be sure to read the comments for some good tips on filtering the form's input too.