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float log ( float $arg [, float $base= M_E ] )

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

arg: The value to calculate the logarithm for
base: The optional logarithmic base to use (defaults to 'e' and so to the natural logarithm).

The logarithm of arg to base , if given, or the natural logarithm.

Версия	Описание
Since 4.3.0	The optional parameter `base` became available. For older versions you can calculate the logarithm in base b of a number n, but using the mathematical identity: log_b(n) = log(n)/log(b), where log is the neperian (or natural) logarithm.