How to customize my checkout_success page?

I'm using PayPal website payments standard and it's working fine.

But would like to customize Zen Cart's checkout_success page.

I know how to edit .html pages but the checkout_success page is a .php file and I have no idea how to tackle this. Or even if it's possible/permissible.

Have looked all over forums and wiki for an answer. Advice appreciated. Thanks in advance.

Most pages in Zencart are created by (at least) 2 files - the PHP file that handles the working stuff and a template file that determines the display of the page. In the case of checkout success, the template file is includes/templates/template_default/templates/tpl_checkout_success_default.php. That's probably the one you want to edit. PHP files can contain PHP as well as HTML.

And remember to save your edited copy in
includes/templates/your_template/templates/tpl_checkout_success_default.php
so you don't lose your modifications when you upgrade.

Thanks Stevesh and gjh. Working well now.