encapsulation.php

<?php
class This
{
   private $data; // Actually protects data enforcing client code to write $obj->getData('foobar');

   public function getData($key) // public not needed, but lets be concise and obvious, code for idiots, simple as that because when you look back at your code in 6 months time you are that idiot!
   {
       if (isset($this->data[$key])) // You will get a notice if you don't check before you do Coding rule #1, check before you presume.
           return $this->data[$key];

       return null;
   }
}
?>