require_once
(PHP 4, PHP 5, PHP 7)
Выражение require_once идентично require за исключением того, что PHP проверит, включался ли уже данный файл, и, если да, не будет включать его еще раз.
Смотри документацию include_once для информации по поведению _once, и чем она отличается от своих не-_once родственников.
Коментарии
If your code is running on multiple servers with different environments (locations from where your scripts run) the following idea may be useful to you:
a. Do not give absolute path to include files on your server.
b. Dynamically calculate the full path (absolute path)
Hints:
Use a combination of dirname(__FILE__) and subsequent calls to itself until you reach to the home of your '/index.php'. Then, attach this variable (that contains the path) to your included files.
One of my typical example is:
<?php
define('__ROOT__', dirname(dirname(__FILE__)));
require_once(__ROOT__.'/config.php');
?>
instead of:
<?php require_once('/var/www/public_html/config.php'); ?>
After this, if you copy paste your codes to another servers, it will still run, without requiring any further re-configurations.
[EDIT BY danbrown AT php DOT net: Contains a typofix (missing ')') provided by 'JoeB' on 09-JUN-2011.]
"require_once" and "require" are language constructs and not functions. Therefore they should be written without "()" brackets!1 - "require" and "require_once" throw a fatal error if the file is not
existing and stop the script execution
2 - "include" and "include_once" throw a warning and the execution
continues
3 - "require_once" and "include_once" as their names suggests ,
they will not include the file if the file was already included with
"require", "require_once", "include" or "include_once"
try the following code:
create a file called "index.php"
<?php
require "first.php"; // this will include the file
include_once "first.php"; // this will not as it was included using "require"
require_once "first.php"; // this will not as it was included using "require"
?>
and another file that is called "first.php" and write the following header
-------------------------------
<h1>Hello every one</h1>
--------------------------------
i hope this will help you
Be careful when using include_once and require_once for files that return a value:
fiddle2.php
<?php
return "Some String";
fiddle.php
<?php
$s = require_once('fiddle2.php');
echo "\n" . $s;
$s = require_once('fiddle2.php');
echo "\n" . $s;
/* output
Some String
1
*/
The second time require_once occurs, it returns 1 because the file has already been included.