WordPress at a glance
function is not described

__autoload() WP 4.6.0

Autoloader compatibility callback.

No Hooks.

Return

Nothing (null).

Usage

__autoload( $classname );
$classname(string) (required)
Class to attempt autoloading.

Code of autoload: wp-includes/spl-autoload-compat.php VER 5.0.3

<?php
function __autoload( $classname ) {
	global $_wp_spl_autoloaders;
	foreach ( $_wp_spl_autoloaders as $autoloader ) {
		if ( ! is_callable( $autoloader ) ) {
			// Avoid the extra warning if the autoloader isn't callable.
			continue;
		}

		call_user_func( $autoloader, $classname );

		// If it has been autoloaded, stop processing.
		if ( class_exists( $classname, false ) ) {
			return;
		}
	}
}