Your Webserver controls which extensions are to be parsed by what language.
Therefore, you need edit httpd.conf (if you have access) or put a .htaccess file in your root directory with the following line.
AddType application/x-httpd-php .php3 .phtml .php .php4 .html .htm
Extensions that can be parsed by PHP are seperated by a space. You will find somehting similiar in httpd.conf if you have access.