Absent materials from Matt Zandstra book "PHP 5 Objects, Patterns, and Practice"

<?php

class Preferences {
    private $props = array();
    private static $instance;

    private function __construct() { }

    public static function setInstance() {
        if ( empty( self::$instance ) ) {
            self::$instance = new Preferences();
        }
        return self::$instance;
    }

    public function setProperty( $key, $val ) {
        $this->props[$key] = $val;
    }

    public function getProperty( $key ) {
        return $this->props[$key];
    }
}

?>