Killzone

Cheat mode:
Start a new game and enter Shooterman as a case-sensitive name. All levels and characters will be unlocked.
intermusic-records.

One shot kill:
Hold L1 at any menu screen, then press Circle, Triangle, Circle, Triangle, X. Note: This code may not work on all levels.
RYUSHOTOKAN.

All movies:
Hold L1 at any menu screen, then press Circle, Square, Triangle, Circle, Square.
ratdt unwetc hawaRYUSHOTOKAN.

Big head enemies:
Hold L1 at any menu screen, then press Circle, Square, X, Circle(2).
RYUSHOTOKAN.

Play as Luger:
Successfully complete mission 2 in story mode.

Play as Rico:
Successfully complete mission 3 in story mode.

Play as Hakha:
Successfully complete mission 4 in story mode.

Quick reload:
Press Reload, switch the gun, then immediately switch back to your original weapon.
Jason Voorhees and Aaron.

When you are in the thick of battle you must reload frequently. When you are low on ammunition or just want to reload (cannot be done with no ammunition loaded), switch to the weapon just before the weapon you want reloaded (before it in the order of weapons you are holding). Then, quickly switch to the weapon you want reloaded while you are getting it out. Your character will load the gun with a fresh clip as he or she preps the gun for use. This will give you a bit more of an edge in battle as you will be reloading much sooner and faster than everyone else.
kilel6.

Fast gun switching:
While sprinting, release L3 then immediately switch your weapon. The next gun will automatically appear in your hands.
Jason Voorhees.

Control character selection screen:
Press Left Analog-stick Left and Left Analog-stick Right at the character selection screen to move the portraits in the corresponding direction.
iidalv.

Hint: Last shots:
It should take about four to seven seconds for a grenade to explode. In that time, shoot like crazy. For easier aim (unless they directly in front of you), zoom by pressing R3.
Rcary3.

Hint: Easy kills:
When in possession of either the Helghast assault rifle or the shotgun, conceal yourself near funnel-points for maximum effect. The shot-spread pattern is enough to inflict mortal damage on dense groups of opponents. Should the survivors retreat; chase them with a hand grenade. You can easily knock out entire "clumps" of opponents using only two actions.
Matt.

When in possession of the sniper rifle, you can take snapshots at "clumps" of enemies. The sniper rifle can eliminate multiple opponents that are "standing-in-line" with a single shot.
Matt.

When playing as Luger, aim for an enemy's chest or head and use her secondary fire. The secondary fire almost always kills an enemy grunt in one shot (depending on where you shoot them). Also, this fire mode is completely silenced so that none of the other enemy units will know where you are. Also, if you cannot find a enemy, use her night vision by pressing Square for an easy shot.
xXxIce35ManxXx.

In the jungle get extremely close to a bush or plants. Zoom in and you will be able to see the enemy very clearly without them seeing you.
Dragn420.

When playing as Luger, pick up a sniper rifle. Zoom with it. If you cannot see your enemy, press Square and use the heat sensor.
Abraham Gawad.

Hint: Defeating the Helghast:
If you play as Rico it is easier to kill the Helghast because of his strong and powerful machine gun.
Flamehead2704.

Hint: Distract enemy Helghast:
When fighting against one or more Helghast, shoot to the side of them. They will look away for a moment. This gives you a chance for a headshot, as they will not move.
DT.

Hint: Sniper pistol:
The sniper pistol is the weapon that Luger uses. Because the sniper rifle has a horrible red glow, use the sniper pistol instead. It has less power, but on secondary fire it can take out an enemy with a head-shot.
Micheal yedziniak.

Hint: Fast gun switching:
While sprinting, release the button. As soon as you do that, switch your gun. The next gun will automatically appear in your hands.
Sterling.

Hint: Shoot three shells with shotgun:
When using the shotgun, it has two modes of fire (regular shoots one shell, alternate shoots two shells). By pressing both fire buttons, you will shoot three shells, which in most cases is more useful. Try using this on groups or stronger enemies.
Aaron.

Hint: Knife special:
If you are Luger or Hakha, when the action symbol appears (when you have your knife out), press X to do a weapon special. Sometimes, you will throw the knife at an enemy's neck, run up, grab it, and rip it out the side.
corey kennedy.

Hint: Assault: Destroying the generators:
When in Battlefields, choose the brown location with a lot of wooden planks and a ruined town in the center of the map. When you play as the ISA, get the chain gun and a backup gun. Run behind the three trees behind their base, where the generators are found. Crouch and hold R1. The chain gun will rapidly fire bullets. The meter will appear like this, approximately.

/--------------\
|=========     |
\______________/

Keep doing this and the first one closest to you will explode. Beware of the explosion.
Xane123.

Hint: Beachhead: Recommended sniper location:
When playing as ISA on this Battlefield level, run out of the small crack in the wall close to the exit from the spawn room. You will find the sniper rifle. Go back into your base and go to the other exit, but move to the large pipes. Between the first two pipes is a doorway. Enter it and climb to the top of a tower with a very good view of the surroundings. There is sniper ammunition at the top of the tower, so there is no need to leave.
Rhys Hall.

Hint: Siska: Triple shot:
Press both fire buttons at the same time to do a triple shot; the same as the shotgun.
Zach LaBrasseur.

Hint: Kill comments:
You can use this trick while playing as Luger after Rico joins on a level.
flyingmonkey58.

Slit someone's throat with the knife and he will say something.

When Hakha joins he may make comments depending on who sees the kill.

Snipe a Helghast with the scoped pistol secondary fire.

Glitch: Standing crouch:
Set your crouch on toggle. While playing, move left to right. Keep repeating this. While moving from left to right, begin tapping Circle. While doing this, tap L2. It will seem that you are standing, but you are really crouching. To get out of it, tap L2 again.
Angie Lynne.

Glitch: Unlimited running:
Sprint and when you start to slow down, rapidly press Circle. This will enable you to keep running while having none left.
Angie Lynne.

Glitch: Chain gun:
In multi-player mode there are endless amounts of weapons that respawn. When the enemy or a friend gets the chain gun, kill them. If you already have it, stand on it. You will be able to shoot three missals at one time.
Raptor.

Glitch: Helghast shoots in air:
Enable the "Big head enemies" code. Kill any Helghast and they will shoot up. Note: Do not watch them shoot into the air if there are any other Helghast in the area. If there are no more Helghast, you can watch him shoot in the air if you can keep up with him.
FireandIce1993.

Glitch: Enemy back or front-flip:
Get a shotgun and triple shot (standard plus secondary fire). Find an enemy (Helghast or ISA). If you fire it into their chest, they will do a backflip because of the force of the gun. Do it to their back for a front flip. This works best with the Battlefields option.
corey kennedy.