|NOTE: This is considered an Expert Only change. This change, if done incorrectly or if there are special circumstances on your site, can disable your site. These instructions are meant as a guideline and further research on your part may be required if this change doesn't result in the desired outcome.|
Using your favorite file editor and FTP application. add the below line to a file called .htaccess in your public_html folder (create the file if it doesn't already exist):
AddType application/x-httpd-php5 .php5 .php4 .php3 .php2 .phtml .php .htm .html
That will cause the server to treat files with the ".html" and ".htm" extension as ".php" files. This is useful if you have moved your site over from another host that was set to parse .html as .php. Note: this can slow your site down since all files are processed through PHP. Use only if neccessary.
If that change results in a 500 error, you may want to replace the above code with:
AddType x-mapp-php5 .php