log()

(PHP 4, PHP 5, PHP 7)

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

参见

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

$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.

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];
}

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.

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))));
  }
?>

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';
?>

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 ;
}
?>

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');

<?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
------------------------------------
*/
?>