PSR-4 is an accepted recommendation that outlines the standard for autoloading classes via filenames. This recommendation is recommended as the alternative to the earlier (and now deprecated) PSR-0.

The fully qualified class name should match the following requirement:

\\<NamespaceName>(\\<SubNamespaceNames>)*\\<ClassName>

• It MUST contain a top level vendor namespace (E.g.: Alphabet)
• It MAY contain one or more sub-namespaces (E.g.: Google\\AdWord)
• It MUST contain an ending class name (E.g.: KeywordPlanner)

Thus the final class name would be Alphabet\\Google\\AdWord\\KeywordPlanner. The fully qualified class name should also translate into a meaningful file path therefore Alphabet\\Google\\AdWord\\KeywordPlanner would be located in [path_to_source]/Alphabet/Google/AdWord/KeywordPlanner.php

Starting with PHP 5.3.0, a custom autoloader function can be defined to load files based on the path and filename pattern that you define.

# Edit your php to include something like:
spl_autoload_register(function ($class) { include 'classes/' . $class . '.class.php';});

Replacing the location (‘classes/’) and filename extension (’.class.php’) with values that apply to your structure.

Composer package manager supports PSR-4 which means, if you follow the standard, you can load your classes in your project automatically using Composer’s vendor autoloader.

# Edit the composer.json file to include
{
    "autoload": {
        "psr-4": {
            "Alphabet\\\\": "[path_to_source]"
        }
    }
}

Regenerate the autoloader file

$ composer dump-autoload

Now in your code you can do the following:

<?php

require __DIR__ . '/vendor/autoload.php';
$KeywordPlanner = new Alphabet\\Google\\AdWord\\KeywordPlanner();