PHP won't "determine" symbols for just any arbitrary base. Because
base_convert() allows conversions for bases between
36 only. Thus, it's not allowed to use it for base 50, for example:
Both frombase and tobase have to be between 2 and 36, inclusive. Digits in numbers with a base higher than 10 will be represented with the letters a-z, with a meaning 10, b meaning 11 and z meaning 35.
I assume that it is about
base_convert() since only this function has direct relation to the question
static char digits = "0123456789abcdefghijklmnopqrstuvwxyz";