A basic PHP autoloader setup

0.autoload_setup.php

<?php
/**
 * In this example, we are autoloading classes within the namespace `Acme\ExampleProject`. The file `index.php` is the 
 * entry point to our project; and we are using the `src` directory to store and organize our PHP class files. Our 
 * autoloader is located in `autoload.php`
 *
 * The basic behavior is illustrated in the fact that PHP will autoload the file `src/mammals/human.php` whenever we 
 * attempt to create a `new \Acme\ExampleProject\Mammals\Human` anywhere in our project's codebase.
 *
 * This diagram illustrates our project structure. Each file's contents can be found below.
 *
 * - /project-root
 * -
 * -   /autoload.php
 * -   /index.php
 * - 
 * -   /src
 * -     /mammals
 * -       /human.php
 *
 * This example does not consider all complications for all use cases and does not necessarily conform to any standard.
 * For more information on standards and best practices, see PHP Standards Recommendations.
 * 
 * @link https://www.php-fig.org/psr/
 */

1.index.php

<?php
/**
 * This file is the entry point to our project
 */

require_once __DIR__ . '/autoload.php';

$john_doe = new \Acme\ExampleProject\Mammals\Human;

var_dump( $john_doe );

2.src\mammals\human.php

<?php

namespace Acme\ExampleProject\Mammals;

class Human {

  /** etc **/
}

3.autoload.php

<?php
/**
 * If the $class being instantiated is within the namespace `Acme\ExampleProject`, then find the correct
 * file within our `src` directory
 */
spl_autoload_register( function( $class ) {

	# namespace prefix that we will use for autoloading the appropriate classes and ignoring others
	$prefix = 'Acme\\ExampleProject\\';

	# base directory where our project's files reside
	$base_dir = __DIR__ . '/src/';

	/**
	 * Does the class being called use our specific namespace prefix?
	 *
	 *  - Compare the first {$len} characters of the class name against our prefix
	 *  - If no match, move to the next registered autoloader in the system (if any)
	 */

	# character length of our prefix
	$len = strlen( $prefix );

	# if the first {$len} characters don't match
	if ( strncmp( $prefix, $class, $len ) !== 0 ) {
		return;
	}

	# get the name of the class after our prefix has been peeled off
	$class_name = str_replace( $prefix, '', $class );

	/**
	 * Perform normalizing operations on the namespace/class string in order to get the file name to be required
	 *
	 * - Replace namespace separators with directory separators in the class name
	 * - Prepend the base directory
	 * - Append with .php
	 */
	$file = $base_dir . str_replace('\\', '/', $class_name ) . '.php';

	# require the file if it exists
	if( file_exists( $file ) ) {

		require $file;
	}

}); # end: spl_autoload_register()