PHP: Hypertext Preprocessor

require_once

(PHP 4, PHP 5, PHP 7, PHP 8)

require_once 表达式和 require 表达式完全相同，唯一区别是 PHP 会检查该文件是否已经被包含过，如果是则不会再次包含。

参见 include_once 的文档来理解 _once 的含义，并理解与没有 _once 时候有什么不同。

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191

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)



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.]

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bobray99 at softville dot com ¶

2 years ago

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.

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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 
       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

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powtac at gmx dot de ¶

9 years ago

"require_once" and "require" are language constructs and not functions. Therefore they should be written without "()" brackets!