Difference between require() and include() in PHP
From PHP manual:-
require() and include() are identical in every way except how they handle failure. include() produces a Warning while require() results in a Fatal Error. In other words, don’t hesitate to use require() if you want a missing file to halt processing of the page. include() does not behave this way, the script will continue regardless. Be sure to have an appropriate include_path setting as well.
So that means, if you want to ensure that file1.php must be read before execute anything else, then you should use require.
If you just want to load a ‘not that important’ template header then you can use include function.