For reference: http://scotty-t.com/2012/07/09/wp-you-oop/

class-pmc-base.php

<?php
abstract class PMC_Base {
  private static $_instance = array();

	private function __construct() {}

	public static function get_instance() {
		$class = get_called_class();
		if ( ! isset( self::$_instance[$class] ) ) {
			self::$_instance[$class] = new $class();
			self::$_instance[$class]->_init();
		}

		return self::$_instance[$class];
	}

	abstract protected function _init();
}

//EOF