I feel like a different extension is clarifying as to the purpose of the file, but you should not use a different extension if the server isn't set up to process it as php.
here is the apache code to execute .inc as php files
Code:
<IfModule mod_mime.c>
AddType application/x-httpd-php .inc
</IfModule>
Alternatively apache also recognizes application/x-httpd-php-source .