two simple PHP scripts that convert to and from piskel format (https://github.com/juliandescottes/piskel) avaiable under MIT License
both scripts should be run from command line
to produce PNG image for each of the layers run
piskel2pngs.php file.piskel
to combine number of pngs files into one piskel file run
pngs2piskel.php frames_number layer_1.png layer_2.png ...
in this case PNGs should have the same dimensions