php code inside any .html in a template

I'm trying to add some php code inside _sub_header.html (or any .html file) in templates/tmpl_uni/_header.html

example : <?php echo "Hello World"; ?> but the "Hello World" doesn't appear.

It's not a web hosting issue because inserting php code in a simple .html works.

in .htaccess :

#AddType application/x-httpd-php5 .html
AddHandler application/x-httpd-php5 .html

Is anyone know what is causing the problem ?

Thanks

php cannot be placed inside the html templates.