Form Processing in PHP

Best I have found for processing forms is
Tectite Form Mail.

Tectite is Secure (spammers can’t use your form to send other emails), written from scratch by professional PHP programmers who understand email security. Your email is hidden in a configuration file (not in the form) all the form has is a name like “Me” or “Sales” (optional, but so important of course you’d use that feature).

Has required fields, value checking, custom error pages (with informative messages, telling the visitor which field they didn’t fill in right).

Send results to text file and/or the email(s) you specify in your configuration file.

Completely free. Written in PHP. Thorough documentation.

Even has a PHP interface. I wrote a form for saving results of online research, used the Tectite PHP interface to save the entire form’s results into a WordPress post.