Q:

AndEngine - General 2D Side-Scroller Problems

I just started using AndEngine, and it's tough to find good tutorials / documentation.
I am making a 2D side-scroller game, where platforms and walls are generated randomly.
The user can control the player's movement (jumping, walking forward).
I have a few general questions:
How do I get a parallax background to scroll only when the player moves?
Should I set the physics world gravity to nothing, and add the force of gravity to each individual object that actually needs it? I don't want gravity to affect floating platforms/walls.
How do I animate a player when it moves (animated walking)?
What is the best way to go about collision detection (player/enemy, player/wall/platform, etc.)?
Sorry, that may have been a lot to ask at once.
Any help is appreciated!

A:

How do I get a parallax background to scroll only when the player moves?

I have never used parallax backgrounds, but I guess that the background moves as the camera moves. If the player doesn't move, the camera shouldn't move so the background won't move either.

Should I set the physics world gravity to nothing, and add the force of gravity to each individual object that actually needs it? I don't want gravity to affect floating platforms/walls.

No, use the gravity. When creating bodies that should be unaffected by forces (Thus immovable) you use the BodyType.StaticBody option. Static bodies can not be moved (Only by code).

How do I animate a player when it moves (animated walking)?

Take a look in the AnimatedSpritesExample. Basically, to animate a sprite it must be instantiated form the AnimatedSprite class and you need to supply it with a TiledTextureRegion (Contains several regions, each with a different image). Then, by calling the animate method of AnimatedSprite class you can start the animation.

What is the best way to go about collision detection (player/enemy, player/wall/platform, etc.)?

If you plan on using Box2D, you can implement a callback called ContactListener which will be called when bodies collide in your PhysicsWorld. 
If you don't plan on using Box2D, you can use AndEngine's methods for collision detection. 
Here is an example. 

