PHP implementation

[mpskovvang]

I've attempted to port your great work to the PHP language: northlands-io/permuteseq

Unfortunately, PHP doesn't support unsigned integers. My knowledge of bitwise operations and shifting is limited, and therefore I'm unsure of the consequences of my signed integers implementation.

I've compared the output with your extension, and (with a modified key scrambling) the results are identical and my tests are passing, but I'm still concerned about integer overflow in edge cases.

Especially these lines, where you're explicit casting to uint64:
https://github.com/northlands-io/permuteseq/blob/33a646f17a680c3589f89f7bdef7e39761548597/src/Permuteseq.php#L48
https://github.com/northlands-io/permuteseq/blob/33a646f17a680c3589f89f7bdef7e39761548597/src/Permuteseq.php#L94
https://github.com/northlands-io/permuteseq/blob/33a646f17a680c3589f89f7bdef7e39761548597/src/Permuteseq.php#L134

I would be very grateful if you could do a short code review.

Thanks,

Martin