Philidor Defense: Repertoire Preparation
Philidor Defense is a very difficult opening for Black. White's main idea is to build a strong center, and wait for Black to make a mistake. Black has a very cramped game, and White can just sit on his position.
Here is a game by Victor Korchnoi.
If Black tries an early ...exd4, then White must capture with Queen, and transpose into Ruy Lopez: Steinitz Defense. The following two games demostrate the ideas.