PHP log() Function

Definition and Usage

The log () function calculates natural logarithm of a number.

Logarithm is inverse of eponential. If 10²=100, it means log₁₀100=2. Natural logarithm is calculated with Euler Number e as base. In PHP, the predefined constant M_E gives value of e which is 2.7182818284590452354

For example, exp(4.60517018599)=100 (it is also same as e^{4.60517018599}=100). Hence, log_e100=4.60517018599

In PHP, log_e is represented by log() function

Syntax

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

Parameters

Return Values

PHP log() function returns logarithm of arg to base. If base is not given, result is natural logarithm.

PHP Version

This function is available in PHP versions 4.x, PHP 5.x as well as PHP 7.x.

Example

Following example calculates natural logarithm of 100

<?php
   $arg=100;
   echo "log(" . $arg . ")=" . log($arg);
?>

Output

This will produce following result −

log(100)=4.6051701859881

Example

Following code calculates natural logarithm of 2. The result is equal to a predefined constant M_LN2−

<?php
   $arg=2;
   echo "log(" . $arg . ")=" . log($arg) . "\n";
   echo "predefined constant M_LN2 : " . M_LN2
?>

Output

This will produce following result −

log(2)=0.69314718055995
predefined constant M_LN2 : 0.69314718055995

Example

Following example calculates log₂1024 and returns 10 −.

<?php
   $arg=1024;
   $base=2;
   echo "log(" . $arg . "," . $base . ")=" . log($arg, $base) . "\n";
?>

Output

This will produce following result −

log(1024,2)=10

Example

Similarly log₂e results in predefined constant M_LOG2E −

<?php
   $arg=M_E;
   $base=2;
   echo "log(" . $arg . "," . $base . ")=" . log($arg, $base) . "\n";
   echo "predefined constant M_LOG2E = ". M_LOG2E;
?>

Output

This will produce following result −

log(2.718281828459,2)=1.442695040889
predefined constant M_LOG2E = 1.442695040889