Why is this an issue?

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.

What is the potential impact?

Including the same file multiple times could have unpredictable results, the _once versions are preferred. Additionally, as 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.

How to fix it

Rewrite your code to only contain require_once and conditional include_once calls.

Code examples

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';