Issues that need to be paid attention to when using include(require) to nest include files in PHP

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* Author:RDP 
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Note: The following instructions are applicable to include, include_once, require, require_once.

The examples are basically in the form of nested inclusions such that a.php contains b.php, and b.php contains c.php. The main discussion is the structural relationship between a.php and c.php. Among them, a.php is entry file.

There are actually three options for the path when including the file .

are respectively

One absolute path                                                                                     ‐ /c.php

Three relative paths (./ is not used in the current directory) Example: ccc/c.php

Regarding the two relative paths, you can intuitively pay attention to the above (2) and ( 3) Differences in relative paths. Having ./ or not makes a big difference when nesting include files! Details are given later.

There is nothing to pay attention to when writing absolute paths directly, but when using

(FILE) When obtaining the absolute path of the current file, please note that No matter whether the php file in which this statement is written is included in other files, dirname(FILE) always points to the location of the php file in which this statement is written. The absolute path .

Example:
a.php as the entry file: require './b/b.php';

b.php: require dirname(FILE).'/c.php';

Then when executing a.php, the dirname(FILE) in b.php gets b.php The absolute path where a.php is located, not the absolute path where a.php is located. (In this example, it means that b.php and c.php belong to the same directory b)

The relative path is based on a certain directory to determine the location of the files that need to be included. The base directory of a relative path is the directory where the entry file for program execution is located, regardless of how many levels of nesting it contains.

Example:

a.php as the entry file: require './b/b.php';

b.php: require './c/c.php'; //Please note that when the c directory is included here, ./ is used

Then, pay attention to the The c directory where c.php is located is at the same level as the b directory, rather than the c directory being below the b directory. Because the inclusion in b.php uses a relative path, and the program entry is a.php, the relative path used in the inclusion of b.php should be based on the directory where the entry file a.php is located.

Three points to note when using relative paths (the current directory does not use ./):

Processed in two steps, first with the program entry file The current directory is used as the basis to search along the relative path. Finding an existing file includes successful exit (same as the above [relative path (the current directory uses ./)]). If it cannot be found, go to the second step. That is, the directory where the php file where the require statement is written is spliced ​​ with the path included in the require. Or based on the directory where the entry file is located, search along the relative path obtained by this splicing. If the file exists, the inclusion is successful. , otherwise it means that the included file does not exist. It is easier to understand by looking at examples.

Example:

a.php as the entry file: require './b/b.php';

b.php: require 'c/c.php'; //Please note that the c directory is not used here./

Then, c/ will first be searched in the directory where the entry file a.php is located. c.php, if present, contains success.

If not found, then splice the path of b.php (./b/b.php) and the path of require in b.php (c/c.php) to get the spliced ​​result The relative path of ./b/c/c.php. Search for ./b/c/c.php in the directory where the entry file a.php is located. If it exists, it means success, otherwise it will cause an error.

/*******************Dividing line***********************/

Let’s briefly summarize the nested inclusion of relative paths.

Nested inclusion of relative paths uses the directory where the program entry file is located as the basis to search for files that need to be included along the relative path.

But if the concept of the current directory is used in the relative path, there will be differences. That is, even if ./ is not used to represent the current directory, the processing will be different.

有./就是以入口文件所在目录为基准来直接搜寻，

没有./，则先以入口文件所在目录为基准来搜寻，找不到的情况下做路径的拼接处理，然后按拼接后的相对路径来搜寻，基准依然是入口文件所在目录。

综合下使用./和不使用./的一个例子：

文件结构如下：

/WEBROOT

——–test.php

——–/folder2/test2.php

——–/folder2/folder3/test3.php

作为程序入口的test.php包含test2.php，而test2.php中包含了test3.php。

那么在test2.php中的包含路径的写法有如下差异：

require ('folder2/folder3/test3.php'); //正确！ 直接在入口程test.php所在的WEBROOT目录下搜寻folder2/folder3/test3.php  
参考上述的【三 使用相对路径（当前目录不使用./）的注意点】

require ('folder3/test3.php');  //正确！ 拼接相对路径然后搜寻  参考上述的【三 使用相对路径（当前目录不使用./）的注意点】

1. require
    (
   './folder2/folder3/test3.php'
   );  
   //正确！  参考上述的【二 使用相对路径（当前目录使用./）的注意点】

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1. require
    (
   './test3/test3.php'
   );    
   //错误！入口程序test.php所在的WEBROOT目录下并不存在test3这个目录！参考上述的【二 使用相对路径（当前目录使用./）的注意点】

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