log

(PHP 4, PHP 5, PHP 7, PHP 8)

log — 自然对数

说明

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

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。

参数

arg

要计算对数的值

base

The optional logarithmic base to use (defaults to 'e' and so to the natural logarithm).

返回值

返回 logbase arg，或者它的自然对数。

更新日志

版本 说明
4.3.0 可选参数 base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

  • log10() - 以 10 为底的对数
  • exp() - 计算 e 的指数
  • pow() - 指数表达式

User Contributed Notes

rhinopete at batpudding dot com 05-Nov-2018 01:28
Seems like unit prefixes should have a standard PHP function.  Maybe in the future.

I found this page while looking for a quick unit prefix function.  The one by  olafurw was voted down, I think because it had unchecked array indexes and /0s.  So here it is fixed and readable.

-Should work down to PHP 4.
-Not meant for fractions or negatives, so anything less than 1 returns 0.
-Not very effective on really really large numbers, but it's easy to add more prefixes.
-Doesn't handle non numeric arguments. PHP 7+ can do this: function binaryprefix( int $units, $unit = '' )

// returns reduced $units with a binary prefix
// ex. ( 110974120, 'B' ) == 105.8MiB
// ex. ( 2^100, 'B' ) == 1048576.0YiB
// ex. ( 0.12314, 'B' ) == 0B
function binaryprefix( $units, $unit = '' )
{
    $prefix = array('', 'Ki', 'Mi', 'Gi', 'Ti', 'Pi', 'Ei', 'Zi', 'Yi');
    $exponent = min( floor( log( max( 1, $units ), 1024 ) ), count( $prefix ) - 1 );

    if ( $units < 1024 )
        return sprintf( '%d%s%s', max( 1, $units + 1 ) - 1, $prefix[$exponent], $unit );
    else
        return sprintf( '%.1f%s%s', $units / pow(1024, $exponent), $prefix[$exponent], $unit );
}

https://en.wikipedia.org/wiki/Binary_prefix#Adoption_by_IEC.2C_NIST_and_ISO

Also more colloquially:
    $prefix = array('', 'k', 'm', 'g', 't', 'p', 'e', 'z', 'y');
ClaudiuS 21-Feb-2013 07:34
If you just need to check if N is a perfect power of Base, log() is SLOW compared to a WHILE construct that will be 2x faster!

Tested on range: 1 ... 20.000.000 => while() is 2.105 times faster

<?php

$number='fill in your number here'
$base='fill in requested base here';

//use when the power is needed
$pow=0;do { $number/=$base;$pow++; } while ($number>1);
if ($number==1) print $base.'^'.$pow;
else print 'False';

//use when just a check is needed
do $number/=$base; while ($number>1);
if ($number==1) print 'True';
else print 'False';
?>
olafurw [at] gmail.com 04-Nov-2008 04:56
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 15-May-2008 07:41
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 05:56
<?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 07:22
$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 03:08
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 01:06
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.