include 'filename.php';
or
require 'filename.php';
Both statements are identical.
INCLUDE file and REQUIRE file statement enable to insert the content of one PHP file into another PHP file, like code , text..
The difference between them is that if errors occur REQUIRE file produce a fatal error and interrupts further display of content.
Although it is easier to use INCLUDE file, it is better to use a REQUIRE file in complex encoding
to avoid errors that can destroy the integrity of the code resulting from the absence of a key file