包含带有Doctype声明的文件

Let's say I have 3 files:

1. index.php - the main file. It has declaration.
2. sources.php - a file that has html code that fetches jquery, bootstrap etc that I'll include in the main (index.php) file. It also has declaration.
3. header.php - a file that has html code that creates the header for the multiple webpages. It will also be included in the main (index.php) file. It also has declaration.

My question is, are the doctype declarations necessary in the sources.php and header.php files?

The code works perfectly fine with them. But, if I only did the declaration in the index file then included the other sources.php file in the head and the header.php right after the opening body tag would it affect my project negatively? Do I really need to have the declaration thrice?

P.S. I know I could just try it by myself but I'm not on my computer and won't be for the next couple of days and I don't want to start installing things on someone else's computer. Anyway, thank you in advance..

You dont need both

1) you keep it in index.php, you dont need it in anything else

2) or you keep it in the templates, index.php has nothing, but index.php cannot output anything before loading the template