.htaccess Query

I wasn't sure where exactly to post this, so I'm just going to post it in the HTML section.Alright, basically what I want to know is if tehre is some sort of .htaccess code i could use on a domain so that a certain file is shown in the header and/or footer of any subdomain?So, for example I added this code and made it include a file Pageheader.php into every subdomain, with the contents I am a test file in it, if i hosted someone as a subdomain, it would show up as "I am a test file" then the rest of their page?also, if this is possible, would there be any way to set what subdomains are exempt from this?