PHP include in a HTML file

I know its been asked before but...

[MizzBia]

Hello everyone,
I'm trying to figure out if there is a way to add a php include into a HTML file...

I found dozens of posts here that say to edit the .htaccess file so that your server parses HTML files as PHP but I have a problem/question with this.

The site is actually serving up all kinds of content using HTML, PHP and Perl/CGI. I have a shopping cart in Perl/CGI which produces SE-friendly pages with a .htm extension. Now, if I make the .htaccess file change, will that affect my cart or how perl/cgi and .htm pages are displayed and parsed? What exactly is the code telling the server to do in this situation?

Thanks in advance for your help

[eelixduppy]

>> What exactly is the code telling the server to do in this situation?

It basically makes each file with a .htm extension (or any extension of your choice for that matter) go through the PHP interpreter before it gets sent to the client. Depending on where your .htaccess is, though, it may not take affect globably.