Create a bijection to get an unordered counter with known max

Question

I have a counter with a known maximum (called max). max can be big (actually it will be either 36^40 - 1 or 62^40 - 1).

I want a bijection b from [0..max] to [0..max] with the following property: b(n+1) not easily guessable from b(n).

I am NOT searching for a cryptographically secure function, I just want as much entropy as possible to obfuscate a little the output of a counter.

The function must be doable in PHP. This allows all the functions PHP does.