Sadly, it looks like this is encoding the ip address differently than the other internet provider was doing.
$longip is translating a value between being in 1.2.3.4. format and being a 32-bit number in the range 0-4294967295. The $base is translating between base-10 and another base, in this case base 16 is the same as case-insensitive hexadecimal. However in your example, you gave a 10-digit string that is not hex. I can see that because 7 of the 10 characters in xS2unZRLae are not part of the hexadecimal alphabet. It also uses both upper and lowercase letters, so it looks like it intends to treat 'x' differently than 'X', while hexadecimal treats "f" and "F" the same.
