How to get the path of a derived class from an inherited method?

How to get the path of the current class, from an inherited method?

I have the following:

<?php // file: /parentDir/class.php
   class Parent  {
      protected function getDir() {
         return dirname(__FILE__);
      }
   }
?>

and

<?php // file: /childDir/class.php
   class Child extends Parent {
      public function __construct() {
         echo $this->getDir(); 
      }
   }
   $tmp = new Child(); // output: '/parentDir'
?>

The __FILE__ constant always points to the source-file of the file it is in, regardless of inheritance.
I would like to get the name of the path for the derived class.

Is there any elegant way of doing this?

I could do something along the lines of $this->getDir(__FILE__); but that would mean that I have to repeat myself quite often. I'm looking for a method that puts all the logic in the parent class, if possible.

Update:
Accepted solution (by Palantir):

<?php // file: /parentDir/class.php
   class Parent  {
      protected function getDir() {
         $reflector = new ReflectionClass(get_class($this));
         return dirname($reflector->getFileName());
      }
   }
?>