Before years ago, if you want to use another .php file from current php file.
You need to include/required before use it to avoid error: : "failed to open stream: No such file or directory".
Example: you're staging on index.php you want to use calendar.php
|__include yourFolder/calendar.php
With PSR4 of composer you don't need to manual include/required as before
2.1 Clone/download IncludeComposer folder to your computer
2.2 Goto IncludeComposer folder > D:\YourFolder\IncludeComposer
2.3 Run composer: composer dumpautoload
2.4 Run file:
|__index.php -- auto include file with composer
|__indexManualInclude.php -- manual include .php files