Try entering these cheats if you're struggling with the game.

Unlock God Mode:
Press L1 + R1 + L2 + R2 + Select + Start.

Unlock ALL Weapons:
Press L1 + R1 + L2 + R2 + Select + Start.

Unlock Unlimited Ammo:
Press L1 + R1 + L2 + R2 + Select + Start.

This is the most difficult setting in the game and is unlocked by completing the 'Hard' difficulty setting in ALL 22 chapters of the game (this must be accomplished without changing to a lower difficulty on any of the chapters). When you have completed this task start a 'New Game' and the option for the 'Crushing' difficulty setting will be available for selection.

During gameplay pause the game and at the 'Pause' screen select 'Bonuses' and then the 'Rewards' option and press Left, Right, Down, Up, Triangle R1, L1, Square and Drake's Baseball T-Shirt will become unlocked.

These are unlocked by going to 'Rewards' and highlighting 'Concept Art' and entering the following codes.

Unlock Concept Art - Bonus 1:
Press L2, Right, Up, Square, Left, Triangle, R1, Down to unlock a video.

Unlock Concept Art Gallery - Bonus 2:
Press Square, L1, Right, Left, Down, R2, Triangle, Up to unlock Bonus Concept Art Gallery 2.

The following features in the game are unlocked by going to the 'Rewards' menu and entering the corresponding code.

Unlock Time's Up Cutscene:
Highlight 'Making a Cutscene - Time's Up' and press L1, Right, Square, Down, Left, Triangle, R1, Up.

Unlock Grave Robbing Cutscene:
Highlight 'Making a Cutscene - Grave Robbing' and press Left, R2, Right, Up, L2, Triangle, Square, Down.

At the 'Costume Select' screen press Left, Right, Down, Up, Triangle, R1, L1, Square to Unlock Nathan Drake's baseball t-shirt which he was wore in the original trailer of the game.