How are you including the external .php files? In MODx, since the templates are stored in the database and are processed through the MODx parser, you cannot use the PHP <?php ... ?> construct. All PHP code must be in a snippet (or a TV with @EVAL binding or a plugin, but those are different matters).
In the Manager, go to Elements->Manage Elements, then the Snippets tab. Click the "New Snippet" link, and put your snippet code there.
include "assets/snippets/mysnippetfiles/whatever.inc.php";
Then in your template, use the snippet tags [!mysnippet!] where you want the output of your snippet to appear.
http://www.sottwell.com/how-snippets-work.html
That said, you should really consider the eForm snippet, which comes with the default installation of MODx. It has good introductory documentation included in the assets/snippets/eform/docs directory.
http://wiki.modxcms.com/index.php/EForm