>but html files usually don't have <?php tags included
(sorry if I didn't expressed correctly what I wanted to say..),
But it is exactly that, what "wasabihowdi" did and what is looking for:
I've done some string processing and stored in a variable in A.php and would like to redirect to B.html and display the result of the string process.
And for this scenario the include() is the perfect solution.
By the way: I also include PHP-Tags into HTML files and use these as templates. As PHP already is a perfect and powerfull template engine and there is no need to use crap things like "Smarty" (what I call a "Template Template Engine Engine"). By simply including() the Templates into PHP all variables are expanded.
For that reason, I use to put all procedural and functional coding into *.php files, and all my templates are stored in *.html.