http://rubysource.com/class-variables-a-ruby-gotcha/

derp.php

<?php
class DemoClass {
    public static $myvar;
  public function __construct() {
    $this::$myvar = "hello world : ";
  }
  public function getMyVar() {
    echo $this::$myvar;
  }
}
class Demo2Class extends DemoClass {
    public function __construct() {
        $this::$myvar = "goodbye world : ";
    }
}
$demo1 = new DemoClass();
$demo1->getMyVar();
$demo2 = new Demo2Class();
$demo2->getMyVar();
$demo1->getMyVar();