[WordPress] Single Design Pattern and Dependency Injection, Part 1

00-container.php

<?php
/**
 * An implementation of the Singleton Design Pattern. Ultimately, this code
 * will be used to show how to leverage the pattern as a very, very simple 
 * container that can be used in dependency injection.
 *
 * @link https://tommcfarlin.com/single-design-pattern-1/
 */
class Container {

	/**
	 * Maintains a private reference to an instance of this class.
	 *
	 * @access private
	 * @var    Container
	 */
	private static $instance;
	
	/**
	 * A private container used for the pattern implementation.
	 *
	 * @access private
	 */
	private function __construct() {}

	/**
	 * The static method used to return an instance to this class.
	 *
	 * @return Container A reference to a static instance of this class.
	 */
	public static function get_instance() {

		if ( ! isset( self::$instance ) ) {
			self::$instance = new self();
		}

		return self::$instance;
	}
}

01-container.php

<?php
/**
 * An implementation of the Singleton Design Pattern. Ultimately, this code
 * will be used to show how to leverage the pattern as a very, very simple 
 * container that can be used in dependency injection.
 *
 * @link https://tommcfarlin.com/single-design-pattern-3/
 */
class Container {

	/**
	 * Maintains a private reference to an instance of this class.
	 *
	 * @access private
	 * @var    Container
	 */
	private static $instance;

	/**
	 * An associative array of objects that are stored in this container.
	 *
	 * @access private
	 * @var    array
	 */
	private static $registry;

	/**
	 * The static method used to return an instance to this class.
	 *
	 * @return Container A reference to a static instance of this class.
	 */
	public static function get_instance() {

		if ( ! isset( self::$instance ) ) {

			self::$instance = new self();
			self::$registry = array();

		}

		return self::$instance;
	}

	/**
	 * A private container used for the pattern implementation.
	 *
	 * @access private
	 */
	private function __construct() {}

	/**
	 * Adds an object to this container.
	 *
	 * @param  string $id       How to uniquely identify the incoming object.
	 * @param  Object $instance A reference to the instance of the object to store.
	 * @throws Exception        If the specified key already exists in the registry.
	 */
	public function add( $id, $instance ) {

		if ( array_key_exists( $id, self::$registry ) ) {
			throw new \Exception( 'This class already exists in the registry.' );
		}

		self::$registry[ $id ] = $instance;
	}

	/**
	 * Retrieves the object represented and stored by the incoming ID.
	 *
	 * @param  string $id The key representing the objected store in the registry.
	 * @return Object     A reference to the instane of the object to use.
	 */
	public function get( $id ) {
		return self::$registry[ $id ];
	}
}