Site templates have to be html, buyers usually customize them using any kind of visual editor. Also using php for common parts (like footer/header) would force them to either install apache/php or constantly upload to their hosting space to preview the changes.
You normally only include php files to handle contact forms and stuff like that
Cheers bitfade exactly what i needed to know.
But including a php version (separated from the html folder) of your template would +1 the quality of your template