Generic autoloader for classes named in WordPress coding style.

class-autoload-wp.php

<?php

if ( ! class_exists( 'Autoload_WP' ) ) {
	/**
	 * Generic autoloader for classes named in WordPress coding style.
	 */
	class Autoload_WP {

		public $dir = __DIR__;

		function __construct( $dir = '' ) {

			if ( ! empty( $dir ) )
				$this->dir = $dir;

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

		function spl_autoload_register( $class_name ) {

			$class_path = $this->dir . '/class-' . strtolower( str_replace( '_', '-', $class_name ) ) . '.php';

			if ( file_exists( $class_path ) )
				include $class_path;
		}
	}
}

Changing the original comment since I found out what was wrong.

This autoloader didn't work for a class which was in a sub folder of my plugin. It ends up appending the 'class' term to the sub folder name and not the actual class file.

You can append the sub folder name that you need in the constructor, to the dir property. And choosing to have your own autoloading class with a different name.