I have created a header using html and javascript, and I want to use that header in the shopping cart. Is it just a matter of "dropping" the code into common/tpl_header.php and blanking out what I don't need?

The header I want to use contains a set of tables with images and a dropdown menu. It is a .htm file so I don't know if that code would jive in a php file and thought I would ask before going ahead ( I have backed up)