Experimenting with singletons for WordPress themes - it makes it easier than dozens of calls to function_exists(), I think. Maybe.

singleton-for-wordpress-themes.php

<?php
/* Child Theme's functions.php */

function collaborate_class_init() {
	class Collaborate extends Collaborate_Parent_Theme {

		public static function theme() {
			if ( ! Collaborate::$instance instanceof self ) {
				Collaborate::$instance = new self();
			}
			return Collaborate::$instance;
		}


		protected function __construct() {
			parent::__construct();
			// init any child specific stuff here
		}
		
		
		public function testing_un_dos_tres() {
			echo 'child';
		}
		
	}
}




/* Parent Theme's functions.php */


class Collaborate_Parent_Theme {
	
	protected static $instance;
	
	public static function theme() {
		if ( ! Collaborate_Parent_Theme::$instance instanceof self ) {
			Collaborate_Parent_Theme::$instance = new self();
		}
		return Collaborate_Parent_Theme::$instance;
	}

	protected function __construct() {
		// init any theme stuff here
	}
	
	public function testing_un_dos_tres() {
		echo 'parent';
	}
	
}



if ( ! function_exists( 'collaborate_class_init' ) ) {
	function collaborate_class_init() {
		
		class Collaborate extends Collaborate_Parent_Theme {

			public static function theme() {
				if ( ! Collaborate::$instance instanceof self ) {
					Collaborate::$instance = new self();
				}
				return Collaborate::$instance;
			}

		}
	}
}




/* Testing shit */




collaborate_class_init();


Collaborate::theme();
function scope_testing() {
	Collaborate::theme();
	Collaborate::theme()->testing_un_dos_tres();
}
scope_testing();