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

Yes. Building on Palantir's answer:

   class Parent  {
      protected function getDir() {
         $rc = new ReflectionClass(get_class($this));
         return dirname($rc->getFileName());
      }
   }

Using ReflectionClass::getFileName with this will get you the dirname the class Child is defined on.

$reflector = new ReflectionClass("Child");
$fn = $reflector->getFileName();
return dirname($fn);

You can get the class name of an object with get_class() :)