When you use the [man]include/man function, PHP will include all code as non-parsed HTML code, unless it reaches PHP tags. So if you have a HTML header, and just do "include('htmlheader.html")" it will include and display as normal HTML.
When a file is included, parsing drops out of PHP mode and into HTML mode at the beginning of the target file, and resumes again at the end. For this reason, any code inside the target file which should be executed as PHP code must be enclosed within valid PHP start and end tags.
Check the HTML page, and see if they have any php opening tags. It's in the HTML page, or a result of the included HTML page.
~Brett