This package, by ChristianQueinnec , fills files as they are first created, with a default content “template”. Each different file extension gets its own template file.
This means every time you create a .js file within emacs, you can fill it with your common header, your common code skeleton. Likewise with creating a new .php file, or a new .java file, etc.
Simple, helpful.
This looks a lot like AutoInsertMode which is a feature of SkeletonMode.