Basic PHP class autoloader that follows the WordPress coding standards for class names and class filenames

autoload.php

<?php

namespace Shea\Example_Plugin;

/**
 * Enable autoloading of plugin classes in namespace
 * @param $class_name
 */
function autoload( $class_name ) {

	/* Only autoload classes from this namespace */
	if ( false === strpos( $class_name, __NAMESPACE__ ) ) {
		return;
	}

	/* Remove namespace from class name */
	$class_file = str_replace( __NAMESPACE__ . '\\', '', $class_name );

	/* Convert class name format to file name format */
	$class_file = strtolower( $class_file );
	$class_file = str_replace( '_', '-', $class_file );

	/* Convert sub-namespaces into directories */
	$class_path = explode( '\\', $class_file );
	$class_file = array_pop( $class_path );
	$class_path = implode( '/', $class_path );

	/* Load the class */
	require_once __DIR__ . '/php/' . $class_path . '/class-' . $class_file . '.php';
}

spl_autoload_register( __NAMESPACE__ . '\autoload' );