Should I use spl_autoload_register() in my plugin?

I am really not sure if it’s good or bad practice to auto loading classes in WP plugin. For me, i see no side effects of using spl_autoload_register (not tested performance)

So to answer your question, I would like to share a class from my upcoming plugin which i am using to auto load classes from a single directory without any issues and it does well for me so far.

/**
 * Annframe Class Autoloader.
 *
 * @package Annframe
 * @since 0.1.0
 */
class Annframe_Autoloader {
    /**
     * Singleton.
     *
     * @since 0.1.0
     * @var Annframe_Autoloader - Single instance.
     */
    private static $_instance = null;

    /**
     * Private Construct.
     *
     * @package Annframe
     * @since 0.1.0
     */
    private function __construct() {
        spl_autoload_register( array( $this, 'load' ) );
    }

    /**
     * Singleton method.
     *
     * @package Annframe
     * @since 0.1.0
     */
    public static function _instance() {
        if ( ! self::$_instance ) {
            self::$_instance = new Annframe_Autoloader();
        }
        return self::$_instance;
    }

    /**
     * Class Loader.
     *
     * @package Annframe
     * @since 0.1.0
     *
     * @param string $class_name - Class name to load.
     * @return null - Do not return anything.
     */
    public function load( $class_name ) {
        $file = str_replace( '_', '-', strtolower( $class_name ) );
        $file="class-" . $file;
        if ( is_readable( trailingslashit( YOUR_PLUGIN_PATH. '/classes-dir' ) . $file . '.php' ) ) {
            include_once trailingslashit( YOUR_PLUGIN_PATH. '/classes-dir' ) . $file . '.php';
        }
        return;
    }
}

Annframe_Autoloader::_instance();

To break this simple class in parts as you see i am using Singleton pattern. Constructor is private and instance() & $_instance belongs to the pattern. Constructor has spl_autoload_register function.

spl_autoload_register( array( $this, 'load' ) );

which is calling load method from self class.
The first two lines from this method is:

$file = str_replace( '_', '-', strtolower( $class_name ) );
$file="class-" . $file;

which is pretty straight. if you follow WPCS it encourages you to follow a class naming convention prefixed with word class and then class name. ofcourse any underscore (_) is replaced with (-) dash.

so a file name of class WPSE_Post would be class-wpse-post.php

Lower casing class names with strtolower and using str_replace to replace underscores with dashes. so WPSE_Post now becomes wpse-post. finally adding a prefix class- on next line.

i am using is_readable in conditional statement which is exchangeable with file_exists. assuming YOUR_PLUGIN_PATH is plugin’s base path and classes-dir is under main plugin dir holding all of your classes that needs autoloading .

include_once is used to load the actual file on call.

Usage:

you just need to include the above auto loader class in your plugin’s base file

/**
 * Class autoloader.
 */
if ( ! class_exists( 'Annframe_Autoloader' ) ) {
    include_once YOUR_PLUGIN_PATH/class-annframe-autoloader.php';
}

and then call your classes on demand.

new XYX_Class();
Another_Class::instance(); // etc

Note: I am not using namespace method in my solution so it may or may not fit according to your needs but posted here in hope that someone atleast get benefits of loading classes dynamically.