It is the bug. You could fix your code like this (PHP > PHP5.3):
class MyClass {
private static $instance;
private function __construct() {
}
static function getInstance() {
if (isset(self::$instance)) {
return self::$instance;
} else {
self::$instance = new static();
return self::$instance;
}
}
}
class ExtendedClass Extends MyClass {
}