log(x,base) returns the logarithm of x when the logarithmic base is base.

<?PHP
	echo log(2,4);  //0.5
	echo log(2,8);  //0.33333333333333
	echo log(2,1);  //0
	echo log(8,2);  //3
?>

If base if not provided, then returns the natural logarithm of x.

<?PHP
	echo log(8);  //2.0794415416798
	echo log(1.2);  //0.18232155679395
?>

log10(x) function returns the Base-10 logarithm of x, log₁₀x.

<?PHP
	echo log10(10);  //1
	echo log10(1000);  //3
	echo log10(0.01);  //-2
?>

log1p(x) returns the natural logarithm of x + 1, log(1+x). This is more accurate when x is almost equal to zero.

<?PHP
	echo log1p(10);  //2.3978952727984
	echo log1p(1E-20);  //0
	echo log(1E-20);  //-46.051701859881
?>