Math 函数
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log

(PHP 4, PHP 5)

log — 自然对数

说明

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

如果指定了可选的参数 base，log() 返回 logbase arg，否则 log() 返回参数 arg 的自然对数。

Note:

参数 base 自 PHP 4.3.0 开始可用。

你可以计算任意以 b 为底 n 的对数，但其实使用的是数学等式：logb(n) = log(n)/log(b)，其中 log 是自然对数。

参见 exp()。

Math 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 自然对数

用户评论:

olafurw [at] gmail.com (04-Nov-2008 12:56)

For those interested. Works with older than 4.3 versions.

<?php
    function byteConvert($bytes)
    {
        $s = array('B', 'Kb', 'MB', 'GB', 'TB', 'PB');
        $e = floor(log($bytes)/log(1024));
     
        return sprintf('%.2f '.$s[$e], ($bytes/pow(1024, floor($e))));
    }
?>

dingus_76 at hotmail dot com (16-May-2008 03:41)

well i been pulling my hair out trying to get log to work with big numbers and i ended up writing a bclog function so to save everyone else the stress here it is

<?php
function bclog($X,$base=10,$decimalplace=12){
    $integer_value=0;
    while($X < 1){
        $integer_value = $integer_value - 1 ;
        $X = bcmul($X , base);
    }
    while($X >= $base){
        $integer_value = $integer_value + 1;
        $X = bcdiv($X , $base );
    }
    $decimal_fraction = 0.0;
    $partial = 1.0;
    # Replace X with X to the 10th power
    $X = bcpow($X , 10);
    while($decimalplace > 0){
        $partial = bcdiv($partial , 10);
        $digit=0;
        while($X >= $base){
              $digit = $digit + 1;
              $X = bcdiv($X , $base);
        }
        $decimal_fraction = bcadd($decimal_fraction , bcmul($digit , $partial));
        # Replace X with X to the 10th power
        $X = bcpow($X , 10);
        $decimalplace = $decimalplace - 1 ;
    }
    return $integer_value + $decimal_fraction ;
}
?>

Ulf Wostner (06-Aug-2006 01:56)

<?php

#--------------------------------------------------------
#     How many digits does an integer have?
#--------------------------------------------------------

function digit_count($n, $base=10) {

  if($n == 0) return 1;

  if($base == 10) {
    # using the built-in log10(x)
    # might be more accurate than log(x)/log(10).
    return 1 + floor(log10(abs($n)));
  }else{
    # here  logB(x) = log(x)/log(B) will have to do.
   return 1 + floor(log(abs($n))/ log($base));
  }
}

# Example:  How many decimal digits for 2 to the power 24?
echo digit_count(pow(2, 24));

# Example: How many bits to write 1 billion in binary, last century?

if($country_code  == 'US') echo digit_count(pow(10, 9), 2);
if($country_code == 'UK') echo digit_count(pow(10, 12), 2);

#--------------------------------------------------------
#     Using log to format columns.
#--------------------------------------------------------

# Suppose we have a dynamically generated list of integers,
# and want to present them as a table. The use of log10 in
# our digit_count helps calculate the proper format string.

function print_list_of_ints($ints, $line_width=40) {

  # Apply our digit_count to the max int among ints.
  $field_width = 2 + digit_count(max($ints));

  # Create format string for printf.
  $format = "%${field_width}d";

  $ints_per_line = floor($line_width/$field_width);

  $border = str_repeat("-", $ints_per_line * $field_width);

  echo "\n$border\n";

  foreach($ints as $count => $int) {
    if( $count and ($count % $ints_per_line == 0)) echo "\n";
    printf($format, $int);
  }

  echo "\n$border\n";
}

# To generate an example, here is a basic function
# returning a list of (pseudo) random numbers.

function rands($how_many) {
  for($i=0; $i < $how_many; $i++) $rands[] = rand();
  return $rands;
}

# Example:  A list of random ints dynamically formatted into columns.

print_list_of_ints(rands(11));

/* Sample output. Numbers and fonts vary. Visualize monospace!

------------------------------------
  1093146637   244503173  1346204527
   638304372   140216732  1054707210
   573915416  1728677954  2038899669
   534854768    12674586
------------------------------------

*/

?>

mcmeijer at yahoo dot com (03-Feb-2005 03:22)

$val = 1000000
$val2 = floor(log($val,10)) gives a value of 5 for $val2 and not 6 as expected.
$val2 = floor(log10($val)) gives the correct value.

c0x at mail dot ru (19-Sep-2004 11:08)

more general version, works fine on negative, very big ($value > 1E+18) and very small ($value < 1E-18) numbers.

function expn($value, $prec = 3, $base = 1000, $prefix = '') {
    $e = array('a', 'f', 'p', 'n', 'u', 'm', '', 'k', 'M', 'G', 'T', 'P', 'E');
    $p = min(max(floor(log(abs($value), $base)), -6), 6);
    return round((float)$value / pow($base, $p), $prec) . $prefx . $e[$p + 6];
}

admin at worldtakeover dot tk (20-Jun-2004 09:06)

In regards to the note about log in base 10 and the round() function. You need to use floor() instead of round() to find out the order of magnitude. That way, you don't have to worry about subtracting 0.5 or whatever.

mightye (at) mightye.org (06-Feb-2003 08:02)

A minor warning:

in PHP < 4.3.0, in order to get the log base 10 of a number, you have to do:
$log10 = log($n)/log(10);

If you want a whole number (to identify the order of magnitude), and you typecast $log10 to (int), you may not get what you expect:
(int)(log(1000)/log(10)) = 2
(log(1000)/log(10)) = 3 (float with no displayed decimal places)

The mathematical error in this causes the typecast to round the result down, even though the error runs out to so many decimal places that it is not displayed, and the float value looks like a whole number.  Instead you may need to do:
round(log($n)/log(10)-0.5,0);

This will give you the order of magnitude of your number.

Presumably in PHP 4.3.0+, a similar result may occur.