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.


== Noncompliant Code Example

----
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
----


== Compliant Solution

----
require_once 'code.php';
if (is_member($user)) {
  include_once $user.'_history.php';
}
require_once 'more_code.php';
----



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