The following script is a single interface for pre-validating any HTML form. It lets you validate your form data the way you want, while handling the rudimentary tasks itself.

1) Change just one variable to make it work with $_GET or $_POST or $_REQUEST.

2) Name which elements are 'required' all at once in a hidden HTML text input.

3) Easily provide field descriptions in hidden HTML text inputs that you can use to print error statements.

4) Call a single function to print out error statements that you can easily style with CSS.

5) It automatically checks if fields marked 'required' have been filled out.

Most of the pre-validation code is from PHP 5: Unleashed by Sams Publishing. I found it extremely useful for a recent project. Sharing it here, I removed some unnecessary functions, and tried to simplify some points.

All the comments are my own; hopefully I provided more than enough explanation.

PHP function to validate an email address.

Fixed missing $ sign.

Validate Email Address

Validate Email Address Against MX

Validate Email Address

Validate Email Address

Fixed the regular expression to check for more strict requirements.

Another solution recommends using a text match pattern.
This works, but as long as you've got mail.jar in your classpath anyway, you might as well use this technique instead. (Plus, I think it works with older JDK 1.2+, whereas pattern.compile is 1.4+)

Note that it also accepts a comma-separated list of emails, just like a To: or CC: address line would.

This is a simple function to check if an email is valid or not.