GD Image Validation..

How much coding is involved for implementing CAPTCHA on an existing php email form? What's involved server side?

ya need to gen a code, use GD to emmbed it in an image, store the info in mysql and then pass a hidden var from the form indicating the mysql record to look up to verify the image code for that user upon posting.

then ya grab the record ID, look up the code and compare it. From there ya throw an error or proceed.

That should be it in a nutshell.

Check that your host supports PHP with GD library any version (built-in PHP 4.3.2 or later version)

You will then only need an HTML or PHP web form and a PHP processing script to process your form data.