See this posting. There was a PHP-Box-Plugin for gpeasy. Typesetter is a multiuser - Cms , and so the developers decided to disable php in the source code of pages.
If You want to change it , You must do it in Your own version ( rmPHP() ) - the developers will not !
For security reasons all php-files begin with define('is_running',true); ! Maybe there are some holes like iframe and ie-sandbox.
Can You publish the source of calculator.php
P.S. It is not possible to uplad php-files except with ftp. User will not be able. So the risk is at the ftp-administrator to do that .
Of course You can always add furter php-functionality with a plugin.