Dynamic HTML templates

Lets say I have a group of templates for my site.
Each template has the same header and footer.
But I want to change the common header and footer without changing every single template.
I can't embed PHP script into the templates ie. include('common_header.php')
I need a way so each page can have a common header that is parsed and a way of doctype switching eg. some pages will use xhtml 1.0 and some will use 1.0.

Any suggestions on the correct way to do this?

Hello,

Parameter "file" enables you to insert the content of a file inside the template. You can use this parameter on a [onload] tag which is processed automatically.

Here is what is usually done:

Sweet thanks Skrol29.
I just realised that there are sub-templates which appears to be a good/proper way of doing it too.