How to use spl_autoload() instead of __autoload()

Question

According to http://php.net/manual/en/language.oop5.autoload.php the magic function __autoload() was deprecated as of PHP 7.2.0 and removed as of PHP 8.0.0.

The official alternative is spl_autoload(). See http://www.php.net/manual/en/function.spl-autoload.php. But the PHP manual does not explain the proper use of this baby.

My question: How to replace this (my automatic class autoloader) with a version with spl_autoload()?

function __autoload($class) {
    include 'classes/' . $class . '.class.php';
}

Problem: I cannot figure out how to give that function a path (it only accepts namespaces).

By the way, there are a lot of threads regarding this topic here on SO, but none gives a clean & simple solution that replaces my sexy one-liner.

Solution

You need to register autoload functions with spl_autoload_register. You need to provide a "callable". The nicest way of doing this, from 5.3 onwards, is with an anonymous function:

spl_autoload_register(function($class) {
    include 'classes/' . $class . '.class.php';
});

The principal advantage of this against __autoload is of course that you can call spl_autoload_register multiple times, whereas __autoload (like any function) can only be defined once. If you have modular code, this would be a significant drawback.

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2018 update to this: there shouldn't really be that many occasions when you need to roll your own autoloader. There is a widely accepted standard (called PSR-4) and several conforming implementations. The obvious way of doing this is using Composer.