Exponentiation is implemented via pow internally, so the result is forced to floating point. Better use the shift operator: $a = (1<<53)+1; ...

Set size of 35e6 has been mentioned, and the desire to push it further. Elements are unique. In decimal notation, certainly more than half of the values are going to be at least eight digits.

Similar concerns hold for the symbols. 35e6 is just about 25 bits, but there would be no problem if the coding weren't sparse to begin with, so let's add a few bits for good measure. Base64 is 6 bits per char, [a-z] is 4.7 bits. In those cases, most of the symbols are going to be no less than five or six characters, respectively.