If the optional base parameter is specified, log() returns logbase arg, otherwise log() returns the natural logarithm of arg.
Note: The base parameter became available with PHP 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().