Singleton Pattern

Singleton Pattern is designed to restrict instantiation of a class to one object. The pattern can be used for global variables or database connection.

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

    // cannot be instantiated
    private function __construct(); 
    
    public static function getInstance() {
        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];
    }
}