PHP or JS - Random heightmap generator

We're looking for a php or javascript (nodejs) function that we can execute that will generate a heightmap image that is easily imported into Unity3D. It will need to generate smooth rolling terrain that is mobile friendly. When performing the random generation the ideal outcome would be that several types of land might be generated: flat, mountainous, hilly, or canyons. At all points of the terrain, it will need to be easily traversable (i.e. NO sharp points). What it will be used for: When a new player starts, the game will make an API call to a backend server and return the random heightmap. This stops folks from cracking the game and generating their own land layout. Attached are examples of what exactly I'm looking for.