You want to take the logarithm of a number.
For logs using base e (natural log), use log( ):
$log = log(10); // 2.30258092994
For logs using base 10, use log10( ):
$log10 = log10(10); // 1
For logs using other bases, pass the base as the second argument to log( ):
$log2 = log(10, 2); // 3.3219280948874
Both log( ) and log10( ) are defined only for numbers that are greater than zero. If you pass in a number equal to or less than zero, they return NAN, which stands for "not a number."
2.7.4. See Also
Documentation on log( ) at http://www.php.net/log and log10( ) at http://www.php.net/log10.