title: Explanation of: register_shutdown_function
body: <p>What does <code>register_shutdown_function</code> do?</p>
  
  <p>I have no understanding whatsoever on the usage of it and what it does. 
  I know you can use it and fit nicely with a custom error handler, but I don't know what to pass through it, what to do with it, etc. </p>
  
  <p>Could someone explain it in the easiest terms for this coding newbie?</p>