title: what shared instance and new instance
body: <p>from flightphp framework documention:</p>
  
  <blockquote>
    <p>By default, every time you load your class you will get a shared
    instance. To get a new instance of a class, simply pass in false as a
    parameter:</p>
  </blockquote>
  
  <pre><code>// Shared instance of the class
  $shared = Flight::db(); 
  
  // New instance of the class
  $new = Flight::db(false);
  </code></pre>
  
  <p>what is shared instance?
  what is difference between these two type in action?</p>