PHP Manual: Add large numbers

Description

void sodium_add(string &$string1, string $string2)

This adds the parameter string2 to string1, overwriting the value stored in string1. This function assumes both parameters are binary strings that represent unsigned integers in little-endian byte order.

Parameters

string1: String representing an arbitrary-length unsigned integer in little-endian byte order. This parameter is passed by reference and will hold the sum of the two parameters.
string2: String representing an arbitrary-length unsigned integer in little-endian byte order.

Return Values

No value is returned.