Include files

require()
include()
require_once()
include_once()

There are 2 basic functions in PHP to include file; require() and include(). These 2 functions are almost identical to each other. require_once() and include_once() just another strict way for require() and include(), which guarantees include files for once.

require() include()

The require() statement includes and evaluates the specific file.

require() includes and evaluates a specific file. Detailed information on how this inclusion works is described in the documentation for include().

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.

<?php
require 'prepend.php';
require $somefile;
require ('somefile.txt');
?>

require_once() & include_once()

The require_once() statement includes and evaluates the specified file during the execution of the script.

This is a behavior similar to the require() statement, with the only difference being that if the code from a file has already been included, it will not be included again.

require_once() should be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.

The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the only difference being that if the code from a file has already been included, it will not be included again. As the name suggests, it will be included just once.

include_once() should be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.