Example class pattern for use in WordPress that takes the best parts of singletons and the best parts of instances and uses them together where 1+1 is greater than 2.

wp-namespace-class.php

<?php

/**
 * Class WP_Namespace_Class
 * 
 * Allows access to the singleton class via the class name, no global or local variable required.
 */
class WP_Namespace_Class {

  /**
   * @var WP_Namespace_Class $_instance;
   */
  private static $_instance;

  /**
   * Initialize 
   */
  static function on_load() {
    /**
     * This could just be the default. We could implement 
     * dependency injection to support subclasses of the 
     * instance class using a variety of means, i.e. via
     * filters, set methods, etc.
     */ 
    self::$_instance = new WP_Namespace_Class_Instance();
  }

  /**
   * Allow access to singleton
   * 
   * @return WP_Namespace_Class_Instance 
   */
  static function get_instance() {
    return self::$_instance;
  }

  /**
   * Returns 'foo'
   * 
   * @return string 
   */
  function get_foo() {
    return self::$_instance->get_foo();
  }

  /**
   * Echo 'foo'
   */
  function the_foo() {
    self::$_instance->the_foo();
  }
  
}
WP_Namespace_Class::on_load();

/**
 * Class WP_Namespace_Class_Instance
 * 
 * This class can be unit tested, easier to work with in a debugger, 
 * supports inheritance and the code just feels better w/o all the statics.
 */
class WP_Namespace_Class_Instance {

  /**
   *
   */
  function __construct() {
    add_action( 'init', array( $this, 'init' ) );
  }

  /**
   * 
   */
  function init() {
    // Initialize hooks and other stuff here.
  }

  /**
   * Returns 'foo'
   * 
   * @return string 
   */
  function get_foo() {
    return 'foo';
  }

  /**
   * Echo 'foo'
   */
  function the_foo() {
    echo $this->get_foo();
  }

}