Autoloading & Namespaces in PHP

This post wil help you understand the What, Why, and How of autoloading, namespaces, and the use keyword in PHP.

If you’re already familiar with the What and Why of autoloading and want to skip right to the How, check out this post on Composer Autoloader: The Easiest Autoloading in PHP.

Before we get into PHP Namespaces, let’s look at Autoloading.

What is Autoloading?

Autoloading is a way to have PHP automatically include the PHP class files of a project.

Consider an OOP PHP project that has more than a hundred PHP classes. How might we make sure that all your classes are loaded before using them? Without autoloading, we might end up manually including every class, like this:


// manually load every file the whole project uses
require_once __DIR__.'/includes/SomeClass1.php';
require_once __DIR__.'/includes/SomeClass2.php';
require_once __DIR__.'/includes/SomeClass3.php';
require_once __DIR__.'/includes/SomeClass4.php';
require_once __DIR__.'/includes/SomeClass5.php';
require_once __DIR__.'/includes/SomeClass6.php';
require_once __DIR__.'/includes/SomeClass7.php';
require_once __DIR__.'/includes/SomeClass8.php';
require_once __DIR__.'/includes/SomeClass9.php';
require_once __DIR__.'/includes/SomeClass10.php';
// ... etc, a hundred more times

$my_example = new SomeClass1();

This is a bit tedious to say the least, and unmaintainable at worst.

What if instead, we could have PHP automatically load class files when we need it? Spoiler alert, we can, and it is called “Autoloading”.

PHP Autoloading 101

To create an autoloader only takes two steps:

  1. Write a function that looks for files that need to be included
  2. Register that function with the spl_autoload_register() core PHP function.

Let’s do this for the above example.

 * Simple autoloader
 * @param $class_name - String name for the class that is trying to be loaded.
function my_custom_autoloader( $class_name ){
    $file = __DIR__.'/includes/'.$class_name.'.php';

    if ( file_exists($file) ) {
        require_once $file;

// add a new autoloader by passing a callable into spl_autoload_register()
spl_autoload_register( 'my_custom_autoloader' );

$my_example = new SomeClass1(); // this works!

There we go. We have no longer need to manually require_once every single class file in the project. Instead, with our autoloader, the system will automatically require the files as their classes are used. For a better understanding of what is going on here, let’s walk through the exact steps in the above code.

  1. We wrote a function named my_custom_autoloader which expects 1 parameter that we have called $class_name. Given a class name, the function looks for a file with that name, and loads that file if found.
  2. spl_autoload_register() is a function in PHP that expects 1 “callable” parameter. A “callable” parameter can be many things, such as a function name, class method, or even an anonymous function. In our case, we provided a function named my_custom_autoloader
  3. We then instantiate a class named SomeClass1 without first having required its php file.

So what happens when this script is run?

  1. First, PHP realizes that there is not yet a class named SomeClass1 loaded, so it begins executing registered autoloaders.
  2. Then, PHP will execute the autoload function we wrote (named my_custom_autoloader), and it will pass in the string SomeClass1 as the value for $class_name.
  3. Our function will define the file as $file = __DIR__.'/includes/SomeClass1.php';, look for its existence (file_exists()), and will require_once __DIR__.'/includes/SomeClass1.php'; if found, resulting in our class’s PHP file being automatically loaded.

Huzzah! We now have a very simple autoloader that will automatically load class files as those classes are instantiated for the first time. In a decent sized project, we have just saved ourselves from writing hundreds of lines of code to include files.

What are PHP Namespaces?

Namespaces are a way to encapsulate like-functionality or properties. A very easy (and somewhat practical) way of thinking of this is like an operating system’s directory structure:

As a concrete example, the file foo.txt can exist in both directory /home/greg and in /home/other, but two copies of foo.txt cannot co-exist in the same directory.

In addition, to access the foo.txt file outside of the /home/greg directory, we must prepend the directory name to the file name using the directory separator to get /home/greg/foo.txt.

The way you define a namespace is at the top of a PHP file, using the namespace keyword:


namespace Jonathan;

function do_something() {
    echo "this function does a thing";

In the above example we have encapsulated the do_something() function within the namespace of Jonathan. This implies a number of things, but most importantly it means that neither of those things will conflict with the other functions of the same name in the global scope.

For example, say we have the above code in its own file name “jonathan-stuff.php”, and in a separate file we have the following code:

require_once "jonathan-stuff.php";

function do_something(){  echo "this function does a completely different thing"; }

// this function does a completely different thing

See, no conflict there. We have 2 functions named do_something() that are able to co-exist with each other. So now all we have to do is figure out how to access the namespaced function. This is done with a syntax that is very similar to a directory structure, with backslashes:


// this function does a thing

The above code is saying, “execute the function named do_something() that resides within the Jonathan namespace”.

This also (and more commonly) is used with classes. For example:


namespace Jonathan;

class SomeClass { }

Which can be instantiated like this:


$something = new \Jonathan\SomeClass();

With namespaces, very large projects can contain many classes that share the same name without any conflicts. Pretty sweet huh?

What is the point of all this? What problems do Namespaces solve?

To answer this question, we need to look back in time to a PHP without namespaces. Previous to PHP version 5.3, we could not encapsulate our classes, there was always a risk of conflicting with another class of the same name.

It was (and still is to some degree) not uncommon to prefix class names resulting in something more like this:


class Jonathan_SomeClass { }

As you may imagine, the larger the code base, the more classes, the longer the prefixes. You shouldn’t be surprised to open an old PHP project and find a class name more than 60 characters long, like:


class Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator { }

“Well, wait a second”, you say, “what’s the difference between writing that versus \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator?”

That’s a great question! The answer lies in the ease of using that class more than once in a given context.

Imagine that we had to make use of a long class name multiple times within a single PHP file. Currently, we have two ways of doing this

Without namespaces:


class Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator { }
$a = new Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator();
$b = new Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator();
$c = new Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator();
$d = new Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator();
$e = new Jonathan_SomeEntity_SomeBundle_SomeComponent_Validator();

Oof, that’s a lot of typing. Well, what about with namespaces?

namespace Jonathan\SomeEntity\SomeBundle\SomeComponent;

class Validator { }

… and elsewhere in the code:


$a = new \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator();
$b = new \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator();
$c = new \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator();
$d = new \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator();
$e = new \Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator();

Well, that certainly isn’t much better. Luckily, there is a third way, leveraging the use keyword to pull a namespace.

Enter, the use keyword

The use keyword “imports” a given namespace into the current context, allowing you to make use of its contents without having to refer to its full path each time.


namespace Jonathan\SomeEntity\SomeBundle\SomeComponent;

class Validator { }

… now we can do this:


use Jonathan\SomeEntity\SomeBundle\SomeComponent\Validator;

$a = new Validator();
$b = new Validator();
$c = new Validator();
$d = new Validator();
$e = new Validator();

Aside from encapsulation, importing is the real power of namespaces.

Now that we have an idea of what both autoloading and namespaces are, let’s combine them together to create a reliable means of organizing our project files.

PSR-4: The Standard for PHP Autoloading & Namespaces

PHP Standard Recommendation (PSR) 4 is a commonly used pattern for organizing a PHP project so that the namespace for a class matches the relative file path to the file of that class.

For example, if you are working within a project that makes use of PSR-4 and you are dealing with a namespaced class named \Jonathan\SomeBundle\Validator();, you can be sure that the file for that class can be found in this relative location in the file system: <relative root>/Jonathan/SomeBundle/Validator.php.

Just to drive this point home, here are some more examples of where a PHP file exists for a class that is within a project making use of PSR-4:

Namespace & Class File Location
\Project\Fields\Email\Validator() <relative root>/Project/Fields/Email/Validator.php
\Acme\QueryBuilder\Where <relative root>/Acme/QueryBuilder/Where.php
\MyFirstProject\Entity\EventEmitter <project root>/MyFirstProject/Entity/EventEmitter.php

Note: this isn’t 100% accurate, as each component of a project has its own relative root, but don’t discount this information–

Knowing that PSR-4 implies the file location of a class will help you easily find any class within very large projects.

How does PSR-4 work?

Another great question! The answer to that question is simple, it’s done with an autoloader function!

Let’s take a look at one PSR-4 example autoloader function to get an idea of how it works:


spl_autoload_register( 'my_psr4_autoloader' );

 * An example of a project-specific implementation.
 * @param string $class The fully-qualified class name.
 * @return void
function my_psr4_autoloader($class) {
    // replace namespace separators with directory separators in the relative 
    // class name, append with .php
    $class_path = str_replace('\\', '/', $class);
    $file =  __DIR__ . '/src/' . $class_path . '.php';

    // if the file exists, require it
    if (file_exists($file)) {
        require $file;

Now let’s walk through what is going on here, assuming we have just instantiated the following class: new \Foo\Bar\Baz\Bug();

  1. PHP executes our autoloader with the $class parameter using a string value of $class = "\Foo\Bar\Baz\Bug"
  2. We use str_replace() to change all backslashes into forward slashes (like most directory structures use), essentially turning our namespaces into a directory path.
  3. We look for the existence of that file in the location <relative root>/src/Foo/Bar/Baz/Bug.php
  4. And finally, we load that file if it is found.

TL;DR – We changed Foo\Bar\Baz\Bug to /src/Foo/Bar/Baz/Bug.php and try to find that file.

Composer – PHP Package manager – Does autoloading for you

Composer is a command line PHP package manager. You may have seen a project before with a composer.json file in its root directory. This file tells Composer about your project, including your project’s dependencies.

Example of a very simple composer.json file:

    "name": "jonathan/example",
    "description": "This is an example composer.json file",
    "require": {
        "twig/twig": "^1.24"

This project is named “jonathan/example” and has 1 dependency of the Twig templating engine (at version 1.24 or higher).

With composer installed on my system I can use this json file to download my project’s dependencies, and in doing so, composer will generate an autoload.php file that will automatically handle autoloading the classes in all your dependencies.

Screenshot of Composer project

If I include this new file in my project, all my dependencies’ classes will be automatically be loaded as I need them in my project!

To re-iterate: Because of the PSR-4 standard and its wide-spread adoption, Composer can generate an autoloader that will automatically handle loading your dependencies as you instantiate them within your project.

Learn more about Creating your own PHP Namespaces and Autoloader with Composer.

Additional resources on used for this post:

2 Thoughts


November 9, 2022

Muchas gracias por esta aportación. Me ha resultado muy útil y esclarecedor.

March 25, 2023

where you say
\”So now all we have to do is figure out how to access the namespaced variables and methods. This is done with a syntax that is very similar to a directory structure, with backslashes:\”

namespace Jonathan;

$a = \'The quick brown fox…\';

function do_something() {
echo \"this function does a thing\";

echo \\Jonathan\\$a;
// The quick brown fox…

From what I\'ve noticed,
echo \\Jonathan\\$a; <—- generates an error

Shouldn\'t it be:
namespace Jonathan;
const a = \'The quick brown fox…\';

This way we can access \"a\" in the namespace with that syntax:
echo \\Jonathan\\a;
// The quick brown fox…

I have found interesting comment on by Lukasz Z.

Well variables inside namespaces do not override others since variables are never affected by namespace but always global:
\"Although any valid PHP code can be contained within a namespace, only four types of code are affected by namespaces: classes, interfaces, functions and constants. \"

Source: \"Defining Namespaces\"

Maybe I\'m missing something? :)


Jonathan Daggerhart
April 29, 2023

You’re absolutely right. Thanks for pointing that out, it’s a pretty big mistake! I’ve updated the post to remove any mention of namespaced variables.

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