The installation of the Trustlogo on PHP pages is the same as detailed at:
Please do not include an html page (with it's own headers) inside an already existing page with full html headers.
Whilst this may work, it will not be cross browser compatible.
Some ppl use template based packages like PHPNuke which basically split off or section each part of the site into it's own template file. Sometimes more than one template file will have it's own <html> tags but these files are never called at the same time and only when some pages require their use, i.e. payment page.
Note: Please check with your software documentation for instructions on how to add custom html headers if required