The Philidor is completely OK. Black has just to understand a few things about the resulting pawn structures, and avoid playing stupid moves like 3...Nc6 (Black wants a PAWN there, else he would play 2...Nc6 instead) and 4...Bg4?
Philidor Defense: Is it worth playing as Black?

Thanks pfren. Can you explain what you mean by 'understand a few things about resulting pawn structures'? Can you upload a game as example?

If White doesn't expect this opening, clearly he doesn't study chess openings at all. I get tired of hearing players parroting that line when discussing chess openings. In this information age you are not going to surprise any serious player at all inside of move 3 or 4. Anyone north of 1800 or so is going to have seen every opening sequence imaginable at one point or another. How deep White's preparation runs in the Philidor as opposed to the Spanish or Scotch or whatever White plays on 2...Nc6 is a different question. But with a few exceptions the lines are not sharp and both players can get by just by - as pfren said - knowing the resulting pawn structures.

Philidor plans may change based on what Black intends to do with his pawn on e5: Will he trade it for the d4 pawn, and try a later central break, or he will keep the pawn on e5 as a stronghold, and try at some phase a queenside expansion?
I would suggest starting your Philidor studies from here:
http://shop.chessbase.com/en/products/the_fighting_philidor
The opening is fine, mostly positional, does not require deep memorization, and also it does have ambitions, since Black keeps many pieces on the board.

i had a buddy about your rating. Guy who loved to sac material like crazy. And he just loved playing the Philidor.
I once asked him why, he answered "Because white makes one wrong move, he can end up in a lot of trouble, fast!"

Thanks everyone! Indeed with a game as old and widely played as chess, objective surprises within first few moves of it! There is a gambit variation in this line: the Philidor Gambit. I wonder if that enjoys as even reputation as the Philidor defense for Black. Simply the options are statistically so high after every move, it gets difficult to come to any final verdict in many situation. But personally, I see I must learn a good deal about it. I only wish if I had access to any standard lesson on it. The DVD suggested by pfren looks tempting, but alas I can not afford it at the moment. I may have to learn it the hard way by actually losing a lot many games!

Chesstempo database shows that 3...Nc6 is most common against 3.Bc4 lines. However, more common is 3.d4 when ...exd4 is most successful for Black (least win rate for white).
I think most modern Philidor players play the pirc move order

I must disagree with the main concept - that the Philidor is much less common than the Spanish. At the low levels of play there are an enormous percentage of players who think either exactly that or that playing the Spanish is too complicated for them. For these reasons, (and possibly others) 2...d6 is played nearly as often as 2...Nc6. Don't be one of those idiots. The Philidor is passive and offers White an easy game even if he has never seen it before. Taking your opponent out of theory is only useful if in the resulting position logical moves aren't strong. It is objectively sound but not practically strong; the latter being of far greater importance in sub-master level play.

best way to play against the philidor is to strangle it and not through complications
I completely agree. Simple, logical developing moves will give White a great position with more space and development than Black. In fact, I can't think of a single advantage that Black possesses (although that doesn't mean that there are actually none).

I respectfully disagree with PFren. The Philidor defense is not "completely ok".
It is possible to reach equality in almost any opening that doesn't drop material, but the task is made more difficult when Black plays passively from the very beginning.
The Philidor is rarely played by top grandmasters because the defensive task is thankless, and rarely results in anything better than a draw for Black.
The Philidor is even worse as an option for players rated below 2000. They often get extremely passive positions and are lost by move 20.
That leaves room for players between about 2000-2400 to play the Philidor successfully. There are a few cute traps in the Philidor that will attract some players, and there are some nice move order tricks that can be used. But this isn't really a great way for anyone with strong aspirations to make GM to play.
Here's probably the most famous game ever played using the Philidor Defense. Black can definitely improve on this game, by playing less aggressively than 3...Bg4? But this game shows the sort of problems Black faces.

The counter-gambit is very dangerous. I've seen many games where black fell to Legal's Mate while trying do a Philidor.

OK, I agree that the Phlidor is "not completely OK". For example in that game Black needed more than twenty moves to win...

Yes, I could show miniature victories to match yours every time. Because, white tends to win more miniatures than Black in the Philidor. Oh, and White tends to score much better against the Philidor than average.
In games between players rated +2500, White scores about 62% (44% wins, 40% draws, 16% losses in my big database).
GMs tend to play the Philidor only in rapid and blitz time controls, but even including those, I only have 296 games in my database between players rated +2500. That is out of more than 187,000 games, not including correspondence games, between such players. That accounts for about .17% of games played by grandmasters. That is, about 1 out of every 600 GM games features a pure Philidor (the position after 1.e4 e5 2.Nf3 d6 3.d4)
Grandmasters, who make professional decisions about their repertoires, generally do not play the Philidor in games that matter.
So, go ahead and bombard us with miniatures. I'm sure the readers here are smart enough to figure out that a few miniatures don't show the whole truth, especially when a) the game is by transposition and b) the winner is more than 100 rating points higher to begin with.

I think the readers are smart enough to not care at all about what the Grandmasters are playing- else they would have to play poisoned pawns, Marshalls and other assorted animals, and lose because they have missed Uzanga's novelty at move 34.
If you want to play like that, then it's OK. Personally, I try avoiding playing that sort of errr, chess, even at engines-on correspondence chess.
I think you are also smart enough to understand this, but you probably messed chess.com with chesspub.com, where most people devote their lives to find a new super dooper novelty at move 27, and then they somehow manage to blunder at move 28.

I must disagree with the main concept - that the Philidor is much less common than the Spanish. At the low levels of play there are an enormous percentage of players who think either exactly that or that playing the Spanish is too complicated for them. For these reasons, (and possibly others) 2...d6 is played nearly as often as 2...Nc6. Don't be one of those idiots. The Philidor is passive and offers White an easy game even if he has never seen it before. Taking your opponent out of theory is only useful if in the resulting position logical moves aren't strong. It is objectively sound but not practically strong; the latter being of far greater importance in sub-master level play.
Black avoids tactics in the opening which usually arise after trying to protect the e5 pawn and this allows him to get developed safely. It isn't intended to 'trick' White. Some players with the White pieces play 1.e4 e5 2.Nf3 Nc6 3.d3 to get Philidor stuff with reversed colors.
The response of Black to 1.e4e5 2.Nf3 is popularly 2...Nc6, however 2...d6 is less popular. This defense bearing the name of Philidor is said as risky and often leads to sharp complications. One of the classical type of risk is evident from the following game where Black is check mated very soon.
The above game has been played many times and still it is common for Black to fall victim. However, my experience in Live Chess with this opening as Black has been a mixed one, winning often, probably because many players playing White do not expect this opening!
Experienced players, please give your comments and share games with this opening as Black! Let us understand if it is worth playing as Black or not!