At root, require, require_once, include, and include_once all perform the same task of
including one file in another. However, the way they perform that task differs, and they should not be used interchangeably.
require includes a file but generates a fatal error if an error occurs in the process.
include also includes a file, but generates only a warning if an error occurs.
Predictably, the difference between require and require_once is the same as the difference between include
and include_once - the "_once" versions ensure that the specified file is only included once.
Because including the same file multiple times could have unpredictable results, the "once" versions are preferred.
Because include_once generates only warnings, it should be used only when the file is being included conditionally, i.e. when all
possible error conditions have been checked beforehand.
include 'code.php'; //Noncompliant; not a "_once" usage and not conditional include $user.'_history.php'; // Noncompliant require 'more_code.php'; // Noncompliant; not a "_once" usage
require_once 'code.php';
if (is_member($user)) {
include_once $user.'_history.php';
}
require_once 'more_code.php';