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Выражение require_once аналогично require за исключением того, что PHP проверит, включался ли уже данный файл, и если да, не будет включать его ещё раз.

Смотрите документацию include_once для информации по поведению _once и чем он отличается от таких же функций, без _once.

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User Contributed Notes 4 notes

bimal at sanjaal dot com
13 years ago
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)

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:

('__ROOT__', dirname(dirname(__FILE__)));

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.]
bobray99 at softville dot com
2 years ago
Be careful when using include_once and require_once for files that return a value:

return "Some String";

= require_once('fiddle2.php');
"\n" . $s;
$s = require_once('fiddle2.php');
"\n" . $s;

/* output
Some String

The second time require_once occurs, it returns 1 because the file has already been included.
boda0128318 at gmail dot com
3 years ago
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

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"


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
powtac at gmx dot de
8 years ago
"require_once" and "require" are language constructs and not functions. Therefore they should be written without "()" brackets!
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