If I'm writing a page that only contains PHP and no HTML (like a page called from a form that sends and email), do I need the HTML, HEAD, and BODY tags?

Or can I just start and end the page as so:

<?php
... content
?>

Short answer:
No

Long answer:
PHP and HTML are only related in that HTML is commonly (but not necessarily) the output of a PHP script.

Thanks for the info. Appreciate it.

If you want it to be valid HTML yes lol.. TO display something on the screen no..

If the output is a HTML webpage then HTML output of the PHP file needs the HTML HEAD and BODY tags, not PHP.

For example you may have a PHP file to be 'pinged' by a payment gateway with the status of the payment. This file would most likely have no output for security reasons and therefore not have to have a HTML, HEAD or BODY tag.