How do I include other files in PHP?
When creating sub directories and including files it is always simpler to use absolute file paths.
The path with reference to root directory is called absolute (https://www.website.com/modules/header.php), you can even remove the domain and just have /modules/header.php. The path with reference to current directory is called relative (../images/phone.png). The ../ indicates that the URL points to the directory above the current folder.
Please see answers relating to a similar question here: difference-between-relative-path-and-absolute-path-in-javascript