This is the plugin autoloader that I use. Of course, customize it as you see fit.

snippet.php

<?php
// Download this snippet as a plugin and more at: https://wordbits.io/?post_type=snippet&p=527
// Created by: jeffmatson
?>

<?php
if ( ! defined( 'ABSPATH' ) ) { exit; }
 
function example_autoload( $classname ) {
    $class     = str_replace( '\\', DIRECTORY_SEPARATOR, str_replace( '_', '-', strtolower( $classname ) ) );
    $file_path = WP_PLUGIN_DIR . DIRECTORY_SEPARATOR . $class . '.php';
    if ( file_exists( $file_path ) ) {
        require_once $file_path;
    }
}
 
spl_autoload_register('example_autoload');