Difference between "include" and "require" in php
require will throw a PHP Fatal Error if the file cannot be loaded. (Execution stops)
include produces a Warning if the file cannot be loaded. (Execution continues)
Here is a nice illustration of include and require difference:
From: Difference require vs. include php (by Robert; Nov 2012)
You find the differences explained in the detailed PHP manual on the page of require:
requireis identical toincludeexcept upon failure it will also produce a fatalE_COMPILE_ERRORlevel error. In other words, it will halt the script whereas include only emits a warning (E_WARNING) which allows the script to continue.
See @efritz's answer for an example
Use include if you don't mind your script continuing without loading the file (in case it doesn't exist etc) and you can (although you shouldn't) live with a Warning error message being displayed.
Using require means your script will halt if it can't load the specified file, and throw a Fatal error.