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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    用户贡献的备注 4 notes

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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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    14
    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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    9
    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
    up
    15
    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!
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