Thanks for your time, nice analyze!
I think, it would be harder to analyze that if there is no obfuscator source code that mention those clues.
Btw, I also made a decompiler for it, you maybe interested https://gist.githubusercontent.com/ammarfaizi2/65495b1d2e8fb7ca3d15290932a8c471/raw/83ea8481717717cf067700da645ae3e21bdffe34/integral_decompiler.php