"use php"

PHP is often mixed into files that contain other languages. Even in the example, the PHP is in a JavaScript file. How does “use php” alone tell the system what is PHP and what might be another language.

It would be nice to be able to use PHP in a file that has the file extension for the primary code it contains, like HTML, but it still seems like there needs to be a way to define where one language ends and other begins.