Difference between array_map, array_walk and array_filter

What exactly is the difference between `array_map`, `array_walk` and `array_filter`. What I could see from documentation is that you could pass a callback function to perform an action on the supplied array. But I don't seem to find any particular difference between them. Do they perform the same thing? Can they be used interchangeably? I would appreciate your help with illustrative example if they are different at all.