(PHP 3, PHP 4, PHP 5)

log - Natural logarithm


float log ( float arg [, float base] )

If the optional base parameter is specified, log() returns logbase arg, otherwise log() returns the natural logarithm of arg.

Замечание: 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.

Смотрите также exp().

Описание на ru2.php.net
Описание на php.ru