How does the PHP autoload work?

I know this issue is kinda old and everybody already wrote something about it, but I think it’s worth to talk about it one more time. This post is for the PHP beginners :)

PHP source files are just plain text files with the php extension, or not even it, if you don’t want, but using an extension helps people identify the filetype and also helps text editors and IDEs to know what syntax highlight to use.

That said, is know that we can use some funcions to include code from other files in a file we are working on. These are the functions:

  • include
  • include_once
  • require
  • require_once

Now, imagine we have this file tree:

  • classes/
    • User.php
  • index.php

In the file classes/User.php there is the class User and the file index.php is the application’s entry point.

If we want to use the class Users in index, we should do something like this:


include "classes/User.php";

$u = new User();

No problems so far, it’s just one class after all. But if we need four classes do be loaded:


include "classes/User.php";
include "classes/Customer.php";
include "classes/Order.php";
include "classes/Invoice.php";

$u = new User();

Ok, now it’s a boring job to do. Actually, this is a job for the autoloader. This feature does exactly what the name says, it automatically loads a class when it is needed.

Setting up an autoloader is pretty simple. Just create a funcion that receives a class name do load and do the inclusion of it’s file. Look at this example:


function my_loader($class)
    $file = __DIR__ . "/classes/{$class}.php";

    if (is_file($file)) {
        include $file;


$u = new User(); // File will be autoloaded here

Simple, right? I created the function my_loader() and it receives a class name to load. Inside the function I created a variable with the full path to the class’ file. Then I just check up if the file exists to include it.

Then I used the spl_autoload_register() function to let PHP know that it can also use my my_loader() to find a class.

Cool, right? And pretty simple, too.

Of course this is a pretty simple way to use an autoloader and nowadays frameworks have their own autoloaders and all you have to do is keep your files in the right places and everything will just work.

Other well know autoloader is the composer’s autoloader.

There are also the PHP comunity’s autoload standards. The most used are PSR-0 which is kinda deprecated, and PSR-4, which is the most recent and accepted standard. So if you want to know how to load the class Awesome\Service\Twitter, just check out the PSR-4 standard.

What about you? Do you have any tips about PHP autoloaders? Share the love in the comments.


Evaldo Junior

Web developer, writer, speaker, Free and Open Source Software contributor and sometimes a gamer and a guitar and ukulele player.

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