Be careful about php require_once usage and relative directories_PHP Tutorial

The article introduces the usage of require_once that everyone often encounters in PHP development. Friends in need can refer to it.

wwwroot //Website root directory. The absolute path is: F:/wwwroot

-- folder_a // Folder A

file_a_a.php

file_a_b.php

file_a_c.php

-- folder_b // Folder B

file_b_a.php

file_b_b.php

file_b_c.php

-- index.php

**************************************************** ******************

This directory hierarchy is already very clear:

wwwroot is the root directory, below which there is the index.php file and two folders folder_a and folder_b

These two folders each have 3 php files

Let’s first look at the contents of the index.php file:

The code is as follows

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代码如下	复制代码
require_once("folder_a/file_a_a.php");      echo "文件folder_a_a.php被包含成功"; ?>

require_once("folder_a/file_a_a.php");

代码如下	复制代码
require_once("../folder_b/file_b_a.php");      $x = new X();      $x.printInfo(); ?>

echo "The file folder_a_a.php was included successfully";

代码如下	复制代码
class X{           function printInfo(){                echo 'success;           }      } ?>

?>

Look at the contents of the folder_a/folder_a_a.php file:

The code is as follows	Copy code
require_once("../folder_b/file_b_a.php"); $x = new X(); $x.printInfo(); ?>

Finally, let’s take a look at the contents of the folder_b/folder_b_a.php file:

The code is as follows	Copy code
class X{ function printInfo(){ echo 'success; } } ?>

ok if I run floder_a/file_a_a.php directly now

Then it will output: success

If I run index.php under wwwroot

Then an error will be reported because the included file cannot be found: file_b_a.php

But if I add dirname(__FILE__).'/'

to all require_once()

Then no matter you run file_a_a.php or index.php, the output will be normal

**************************************************** **********

Question:

The first time I used a relative path, so an error occurred when I included it repeatedly

The second time I used the absolute path, so there was no error. But I am still a little confused:

I first analyzed the following reasons why using relative paths will cause errors:

I run index.php, it can find the folder_a directory and file_a_a.php in that directory, so it copies the contents of folder_a/file_a_a.php to the first line of index.php (including the statement line), and then continue to run (that is, run the included content), so this is equivalent to running require_once('../folder_b/file_b_a.php'); in file_a_a.php in index.php. Find this path file (file_b_a.php) based on the current location of index.php, but of course it can’t be found, so it’s an error.

But isn’t it the same when I use absolute paths? But why doesn’t it go wrong? Maybe everyone is a little confused about this sentence, let me explain it in detail (according to the running order of the program).

The program first runs index.php (note that I added dirname(__FILE__) at this time, so the current path is absolute),

index.php first runs the first line of code: require_once(dirname(__FILE__).'/'.'folder_a/file_a_a.php');

dirname(__FILE__) is f:/wwwroot/, so the path contained in this code is:

f:/wwwroot/folder_a/file_a_a.php

This path is correct, so it’s no problem, right

ok the first step is completed correctly

Then it copies the code in file_a_a.php to this location in index.php:

Then it still continues to run: This is to run all the code in file_a_a.php in index.php, so let’s see what code it runs?

The code is as follows

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代码如下	复制代码
require_once(dirname(__FILE__).'/'."../folder_b/file_b_a.php");      $x = new X();      $x.printInfo(); ?>

require_once(dirname(__FILE__).'/'."../folder_b/file_b_a.php");

代码如下	复制代码
require_once(dirname(__FILE__).'/'."../folder_b/file_b_a.php");      $x = new X();      $x.printInfo();      echo "文件folder_a_a.php被包含成功"; ?>

$x = new X();

$x.printInfo();

?>

代码如下	复制代码
c.php require_once("a.php"); require_once("demo/b.php"); B::demo();a.php class A { }

Yes, that’s it. It should be noted that these codes have been copied to index.php, that is to say, the content of index.php now actually becomes:

The code is as follows	Copy code
require_once(dirname(__FILE__).'/'."../folder_b/file_b_a.php"); $x = new X(); $x.printInfo(); echo "The file folder_a_a.php was included successfully"; ?>

Let’s take a look at some things to note Suppose there are the following three files, c.php a.php b.php and the corresponding storage directory is: localhost/ localhost/ localhost/demo

The code is as follows	Copy code
c.php require_once("a.php"); require_once("demo/b.php"); B::demo();a.php class A { }

The content of b.php is more interesting, because it inherits CLASS A, so I also introduced a.php into it

The code is as follows

代码如下	复制代码
require_once("../a.php"); class B extends A {     public static function demo()     {     echo "xx";     } }

Copy code

require_once("../a.php");
class B extends A {

代码如下	复制代码
require_once("a.php"); require_once("../a.php"); class B extends A {     public static function demo()     {     echo "xx";     } } B::demo();

Public static function demo()

{
echo "xx";

}

}

The system reports an error when executing localhost/c.php. The error message is as follows Warning: require_once(../a.php) [function.require-once]: failed to open stream: No such file or directory in F:wwwdemob.php on line 2

Fatal error: require_once() [function.require]: Failed opening required '../a.php' (include_path='.;C:php5pear') in F:wwwdemob.php on line 2 However, I was surprised to find that if Remove the require_once statement in b.php and the execution is normal. So there must be too many require_once statements defined? The reason is that Class A was redefined twice? But no. If I only add require_once(‘a.php’); to c.php, it will be correct even if I write it twice. So what’s going on?

The reason is that the directory level defined by b.php is inconsistent with the directory level of the c.php execution file, resulting in two require_once statements in c.php. Make it equivalent to

The code is as follows

Copy code

require_once("a.php"); require_once("../a.php"); class B extends A { Public static function demo() { echo "xx"; } } B::demo(); The reason was found, because in c.php, its relative directory ".." is the upper layer of c.php, resulting in an error that the file cannot be found. Therefore, our conclusion is that in PHP, when using require_once, there are different hierarchical relationships and relative directories must be used with caution. require_once is very simple to use, but everyone should try to use absolute paths when using it. http://www.bkjia.com/PHPjc/632204.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/632204.htmlTechArticleThe article introduces the usage of require_once that you often encounter in PHP development. Friends in need can refer to it. one time. wwwroot //Website root directory The absolute path is: F:/wwwroot --...