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PHP : Language Reference : Control Structures : require_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. See the documentation for require() for more information on how this statement works.

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.

For examples on using require_once() and include_once(), look at the » PEAR code included in the latest PHP source code distributions.

Return values are the same as with include(). If the file was already included, this function returns TRUE


require_once() was added in PHP 4.0.1


Be aware, that the behaviour of require_once() and include_once() may not be what you expect on a non case sensitive operating system (such as Windows).

Example 7.11. require_once() is case insensitive on Windows

require_once "a.php"; // this will include a.php
require_once "A.php"; // this will include a.php again on Windows! (PHP 4 only)

This behaviour changed in PHP 5 - the path is normalized first so that C:\PROGRA~1\A.php is realized the same as C:\Program Files\a.php and the file is required just once.


Windows versions of PHP prior to PHP 4.3.0 do not support accessing remote files via this function, even if allow_url_fopen is enabled.

See also require(), include(), include_once(), get_required_files(), get_included_files(), readfile(), and virtual().

Related Examples ( Source code ) » require_once

Code Examples / Notes » require_once


With both of your functions guys, Pure-PHP and jtaal at eljakim dot nl, you'll not have any variables available GLOBALly if they're supposed to be globals...
That's why my import handles better those situation. OK, SOME MAY DISPUTE that using include_once and require_once may slow down an application. But what's the use to do IN PHP what the interpreter *should* do better for you. Thusly these workarounds shall, some time in the future, DIE.
Thus It's better to well design your application to keep some order using few INCLUDES and REQUIRES in it rather than insert MANY AND SEVERAL *_once around.


When you feel the need for a require_once_wildcard function, here's the solution:
<?php // /var/www/app/system/
function require_once_wildcard($wildcard, $__FILE__) {
 preg_match("/^(.+)\/[^\/]+$/", $__FILE__, $matches);
 $ls = `ls $matches[1]/$wildcard`;
 $ls = explode("\n", $ls);
 array_pop($ls); // remove empty line ls always prints
 foreach ($ls as $inc) {
The $__FILE__ variable should be filled with the special PHP construct __FILE__:
<?php // /var/www/app/
require_once_wildcard("classes/*.inc.php", __FILE__);
The (*.inc.php) files inside the directory classes are automagically included using require_once_wildcard.
This solution may not be as useful when using PHP5 in combination with classes and the autoload feature.
Jaap Taal

manuel schaffner

The path for nested require_once() is always evaluated relative to the called / first file containing require_once(). To make it more flexible, maintain the include_path (php.ini) or use set_include_path() - then the file will be looked up in all these locations.


The following only applies to case insensitive systems like Windows.
Even though the documentation sais that "the path is normalized" that doesn't seem to be true in all cases.
If you are using the magic __autoload() function (or if the framework you're using is using it) and it includes the requested class file with complete path or if you override the include path in mid execution, you may have some very strange behavior. The most subtle problem is that the *_once functions seem to differentiate between c:\.... and C:\....
So to avoid any strange problems and painfull debugging make sure ALL paths you use within the system have the same case everywhere, and that they correspond with the actual case of the filesystem. That includes include paths set in webserver config/php.ini, auto load config, runtime include path settings or anywhere else.


require_once() is NOT independent of require().  Therefore, the following code will work as expected:
echo "Hello";
test.php outputs: "Hello".
-- Miqro


require_once can slower your app, if you include to many files.
You cann use this wrapper class, it is faster than include_once

antoine dot pouch

require_once (and include_once for that matters) is slow.
Furthermore, if you plan on using unit tests and mock objects (i.e. including mock classes before the real ones are included in the class you want to test), it will not work as require() loads a file and not a class.
To bypass that, and gain speed, I use :
class_exists('myClass') || require('path/to/myClass.class.php');
I tried to time 100 require_once on the same file and it took the script 0.0026 seconds to run, whereas with my method it took only 0.00054 seconds. 4 times faster ! OK, my method of testing is quite empirical and YMMV but the bonus is the ability to use mock objects in your unit tests.

thomas dot revell

Regarding the case insensitivity problems on Windows, it looks to me as though it is a problem in PHP5 as well (at least in some cases).
The following gave me problems:
From file URLSwitcher.php
require_once 'slimError/slimError.php';
require_once 'Navigator_Cache.php';
From file Navigator_Cache.php
require_once 'slimError/slimerror.php';
From file slimerror.php
class SLIMError {
The above setup gave me an error : "Cannot redeclare class SLIMError"
If I change the require_once in URLSwitcher.php to match the one in Navigator_Cache.php, there isn't a problem, but if I do this the other way round, the same problem occurs.

jaisen -

NOTE: This function changed how it worked.  In PHP 3 this behaved very differently than it does on PHP 4.  Require used to include and parse the file regardless where the require line was positioned.
For example (PHP3):
 if(false){ require_once 'file_does_not_exist.php'; }
That code throw a fatal exception even though it's in a conditional block which evaluates to false.  In PHP 4 the file is never included or parsed, so no exception is thrown.
For example (PHP4)
 if(false){ require_once '1_file_does_not_exists.php'; }
 require_once '2_file_does_not_exists.php';
Stops execution of the script on trying to require the 2nd bypasses the first require.

martijn dot lowrider

How to use Require_Once with error reporting to include a MySQL Connection file:
$MySQLConnectFile = './inc/MySQL.Class.php';
if ( is_dir ( './inc/' ) )
$IncIsDir == TRUE;
if ( file_exists ( $MySQLConnectFile ) )
$MySQLFileExists == TRUE;
if ( $IncIsDir && $MySQLFileExists )
require_once ( $MySQLConnectFile )
echo '<b>Error:</b> <i>Could not read the MySQL Connection File. Please try again later.';


Check how many files you are including with get_required_files(). If it's a significant number (> 100), it may be worth "compiling" the main PHP file. By "compiling", I mean write a script that reads a PHP file and replaces any "include/require_once" references with either:
- the file that it's requiring
- a blank line if that file has been included before
This function can be recursive, thus building up a large PHP file with no require_once references at all. The speedup can be dramatic. On one of our pages that included 115 classes, the page was sped up by 60%.


Beware: As miqrogroove said below, require_once() is not independent of require() -- but vice versa, require() IS independent of require_once()!
Let's turn miqrogroove's example around:
echo "42!<br />\n";
require_once 'echo.php';
require 'echo.php';
// 42!
// 42!
So: require_once() will NOT include a file previously included by require(), while require WILL include a file previously included by require_once.


A very interesting behaviour of require_once (and probably all include commands):
consider the following files:
/index.php -> require_once('/inc/library.php');
/function1.php -> print('/function1.php');
/inc/library.php -> require_once('function1.php');
/inc/function1.php -> print('/inc/function1.php');
Note that /function1.php and /inc/function1.php are files with the SAME filename in different folders.
If you "/index.php" is executed it will output
Although /index.php "includes" /inc/library.php the scope of the file is still /index.php therefor /function1.php will be found even it could be asumed the /inc/function1.php is the correct one.
And it gets more interesting: if you delete /function1.php and execute /index.php PHP checks this and "includes" /inc/function1.php.


@georg_gruber at yahoo dot com
Files for including are first looked in include_path relative to the current working directory and then in include_path relative to the directory of current script. E.g. if your include_path is ., current working directory is /www/, you included include/a.php and there is include "b.php" in that file, b.php is first looked in /www/ and then in /www/include/. If filename begins with ./ or ../, it is looked only in include_path relative to the current working directory.
Taken from


function & rel($r, &$f) {return file_exists( ( $f = ( dirname($r).'/'.$f ) ) );}
function & relf($r, $f) {return rel($r,$f) ? file_get_contents($f) : null;}
function & reli($r, $f) {return rel($r,$f) ? include($f) : null;}
function & relr($r, $f) {return rel($r,$f) ? require($f) : null;}
function & relio($r, $f) {return rel($r,$f) ? include_once($f) : null;}
function & relro($r, $f) {return rel($r,$f) ? require_once($f) : null;}
I found it useful to have a function that can load a file relative to the calling script and return null if the file did not exist, without raising errors.
Load file contents or return blank if it's not there.
Relative to the file calling the function.
echo relf(__FILE__, 'some.file');
It was easy to modify and just as useful for require/include.
Require the file once.
It's like suppressing error messages with @ but only when the file does not exist.
Still shows compile errors/warning, unless you use @relro().
Relative to the file calling the function.
relro(__FILE__, 'stats.php');
If you work with a deep php file structure and a barrage of includes/requires/file-loads this works well.

18-mar-2004 05:49

> Mac OS X systems are also not case-sensitive.
That depends on the filesystem:
- HFS and HFS+ are NOT case sensitive.
- UFS is case sensitive.

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