Variables available on display suite php field
You can get access to the node via the $entity variable within the "Field code" text area.

This will only work for the Display Suite "Code Field". If you are using Display Suite 7.x-2.x, you'll have to enable the "Display Suite Format" module.

If anything is setup properly, you should see this help text below the "Field code" text area:


  You may post Display Suite code. You should include  tags when
  using PHP. The $entity object is available.