title: What are static and dynamic variables / methods in OOP?
body: <p>I am trying to better understand basic concepts in OOP. 
  What are static and dynamic variables and methods in object-oriented programming?
  What is, for instance, the difference between using $this vs. double colon (::)?</p>
  
  <h3>$this ($this->a_method())</h3>
  
  <ul>
  <li>Advantages: ?.</li>
  <li>Disadvantages: ? ... "this" is not self-documenting as in: <code>$this->method_from_an_extended_class()</code>.</li>
  </ul>
  
  <h3>Double colon (someclass::a_method())</h3>
  
  <ul>
  <li>Advantages: ?</li>
  <li>Disadvantages: ?</li>
  </ul>