log Natural logarithm floatlog floatarg floatbase If the optional base parameter is specified, log returns logbase arg, otherwise log returns the natural logarithm of arg. The base parameter became available with PHP version 4.3.0. As always you can calculate the logarithm in base b of a number n, but using the mathematical identity: logb(n) = log(n)/log(b), where log is the neperian (or natural) logarithm. See also exp.