What is __ dir __ in php?

Summary: in this tutorial, you’ll learn about how to use the PHP __DIR__ magic constant when including a PHP file.

Introduction to the PHP __DIR__ magic constant

PHP 5.3 introduced a new magic constant called __DIR__. When you reference the __DIR__ inside a file, it returns the directory of the file. The __DIR__ doesn’t include a trailing slash e.g., / or \ except it’s a root directory.

When you use the __DIR__ inside an include, the __DIR__ returns the directory of the included file.

Technically speaking, the __DIR__ is equivalent to the dirname(__FILE__). However, using the __DIR__ is more concise than the dirname(__FILE__).

Simple PHP __DIR__example

Suppose you have the following directory structure:

.
├── inc
│   ├── footer.php      
│   └── header.php
└── index.php
Code language: CSS (css)

The header.php contains the code of the header part:

"en">

    "UTF-8">
    "viewport" content="width=device-width, initial-scale=1.0">
    

Code language: PHP (php)

The footer.php contains the code of the footer part. It also shows the value of the __DIR__ constant:

 echo __DIR__ ?>


Code language: PHP (php)

The index.php includes both header.php and footer.php files. It also shows the value of the __DIR__ constant:

 require 'inc/header.php' ?>
Home

 echo __DIR__ ?>
 require 'inc/footer.php' ?>
Code language: PHP (php)

The following shows the output on the web browser:

Home
C:\xampp\htdocs\web
C:\xampp\htdocs\web\inc
Code language: PHP (php)

The __DIR__ in the index.php returns the current directory of the index.php C:\xampp\htdocs\web while the __DIR__ in the footer.php file returns the directory of the footer.php file C:\xampp\htdocs\web\inc.

Why using PHP __DIR__

Suppose that you have a project with the following directory structure:

.
├── admin
│   └── dashboard
│       └── index.php
├── config
│   └── app.php
├── inc
│   ├── footer.php
│   └── header.php
└── public
    └── index.php



Code language: CSS (css)

The config/app.php contains the application’s configuration:

define('APP_NAME', 'My Awesome App');
Code language: PHP (php)

The header.php contains the header part of a page. It also includes the config/app.php file and uses the APP_NAME constant:

 require '../config/app.php' ?>

"en">

    "UTF-8">
    "viewport" content="width=device-width, initial-scale=1.0">
    

Code language: PHP (php)

The footer.php contains the closing tags of the body and html elements:

Code language: PHP (php)

The public/index.php file includes both header.php and footer.php files:

 require '../inc/header.php' ?>
Home

 require '../inc/footer.php'?>
Code language: PHP (php)

If you navigate to the http://localhost/web/public/index.php, you’ll see the following output when you view the source of the page:

"en">

    "UTF-8">
    "viewport" content="width=device-width, initial-scale=1.0">
    


    
Dashboard


Code language: PHP (php)

So the index.php page in the public directory works as expected.

The index.php in the admin/dashboard also includes the header.php and footer.php files:

 require '../../inc/header.php' ?>
Dashboard

 require '../../inc/footer.php' ?>



Code language: PHP (php)

When you browse the page http://localhost/web/admin/dashboard/index.php, you’ll see the following error:

Warning: require(../config/app.php): failed to open stream: No such file or directory in C:\xampp\htdocs\web\inc\header.php on line 1

Fatal error: require(): Failed opening required '../config/app.php' (include_path='\xampp\php\PEAR') in C:\xampp\htdocs\web\inc\header.php on line 1
Code language: PHP (php)

The error message shows that the header.php cannot load the '../config/app.php' file.

When the index.php in the admin/dashboard directory loads the code from the header.php, it’ll find the config/app.php inside the admin directory, not root directory.

Since the admin directory doesn’t have the config/app.php file, PHP issues an error.

To fix this issue, you can use the __DIR__ when you include the config.php in the header.php file like this:

  require __DIR__. '/../config/app.php' ?>
Code language: PHP (php)

The index.php in the admin/dashboard will work correctly.

This is because when PHP loads the inc/header.php file from either public/index.php or admin/dashboard/index.php, the __DIR__ inside the inc/header.php always returns C:\xampp\htdocs\web\inc.

Therefore, it’s a good practice to use the __DIR__ constant when you include a file.

Summary

• PHP __DIR__ returns the directory of a file or the directory of the include file when the file is used as an include.
• Use __DIR__ when you include a file.

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