Math 函数
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pow

(PHP 4, PHP 5)

pow — 指数表达式

说明

number pow ( number $base , number $exp )

返回 base 的 exp 次方的幂。如果可能，本函数会返回 integer。

如果不能计算幂，将发出一条警告，pow() 将返回 FALSE。PHP 4.2.0 版开始 pow() 不要产生任何的警告。

Note:

PHP 不能处理负数的 base。

Example #1 pow() 例子

<?php

var_dump(pow(2, 8)); // int(256)
echo pow(-1, 20); // 1
echo pow(0, 0); // 1

echo pow(-1, 5.5); // error

?>
Warning

在 PHP 4.0.6 及之前版本中，pow() 总是返回 float，并且不产生警告。

参见 exp()，sqrt()，bcpow() gmp_pow() 和。

Math 函数
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PHP手册 - N: 指数表达式

用户评论:

tagg_maiwald at yahoo dot com (02-Nov-2010 04:34)

This function returns the value of a positive base with a signed floating point exponent:
function sf_exp( $fl_x = 1, $fl_y = 0)
{    $fl_exp = 0.0;
    if (0 > $fl_x)
    {    // Alter this logic container to enable processing of negative bases.
        $fl_exp = -1.0;
    } else
    {    $bool_neg = (0 > $fl_y);

        if ($bool_neg)
        {    $fl_y = 0 - $fl_y;
        }
   
        $fl_xlog = log10( $fl_x);
        $fl_xylog = ( $fl_xlog * $fl_y);
        $fl_exp = pow( 10, $fl_xylog);

        if ($bool_neg)
        {    $fl_exp = 1/$fl_exp;
        }
    }
   
    return $fl_exp;
}

Anonymous (25-Apr-2010 10:24)

You can increase the 'precision' php.ini setting a little to work with larger float numbers here, but this comes at at cost of sacrificing decimal accuracy.  The default 'precision' is 14.  5 is about the threshhold that php can handle for decimal accuracy before at least some number corruption starts showing or it cannot output the actual number, and 16 for large number accuracy, as demonstrated by throwing this into the table below:
<?php
    echo "<td>".pow(10, $i) - 1)."</td>";
?>

See the table below for an example, and adjust your php.ini 'precision' setting according to what your OS and PHP version can handle and what number size you want to work with.  Alternatively, you can use the bc math functions for more accuracy all around, and not have to rely on the 'precision' ini setting at all, but this moves out of the realm of strictly floats and into strings.

Also, PHP just prefers to display the 'E' notation of float values where possible after about 5 decimal places rather than the actual decimal number (1.0E-5 vs 0.00005).

<?php

echo "<table>";
for($i = 0; $i < 50; $i++) {
    $precision = $i + 1;
    ini_set('precision', $precision);
    echo "<tr>";
    echo "<td>".$precision."</td>";
    echo "<td>".pow(10, $i)."</td>";
    echo "<td>".pow(10, (-1 * $i))."</td>";
    echo "<td>".bcpow('10', (string) $i, $precision)."</td>";
    echo "<td>".bcpow('10', (string) (-1 * $i), $precision)."</td>";
    echo "</tr>";
    }
echo "</table>";

?>

Bottom line though is, if you're working with larger numbers or require very fine decimal precision or prefer displaying the full decimal number, use the bc math functions instead.  And, do check out PHP's considerations about the float type here: http://php.net/manual/en/language.types.float.php

Matt Dudley (16-Jul-2008 11:14)

Calculate wind chill based on the National Weather Service formula.

$temp = 25;
$wind_speed_mph = 6;

$wind_chill = 35.74+(.6215*$temp_f)-(35.75*(pow($wind_speed_mph, 0.16)))+(.4275*$temp_f*(pow($wind_speed_mph, 0.16)));

Value only valid when the temp is 45 or below.... I used this with a weather script I wrote that reads an xml file. They don't provide wind chill.

Docey (04-May-2007 06:33)

no integer breaking here, pow just silently switches to using floats instead of integers.

pow(2, 31) = integer value
pow(2, 32) = float value.

the manual says the limit for floats is machine dependent so i did a little loop to see how far it will go before becomming infinit. the result is 1023.

pow(2, 1023) = float
pow(2, 1024) = ifinit.

tested on php 4.4.1 under windows2000 on an AMD AthlonXP 2800+.

gilthansREMOVEME at gmail dot com (15-Dec-2006 03:50)

Note that pow(0, 0) equals to 1 on PHP 4 (only tested it there), although mathematically this is undefined.

moikboy (nospam) moikboy (nospam) hu (10-May-2006 09:27)

Here is a function for calculating the $k-th root of $a :

<?php
function root($a,$k){return(($a<0&&$k%2>0)?-1:1)*pow(abs($a),1/$k);};
?>

louis [at] mulliemedia.com (31-Dec-2004 04:02)

Here's a pow() function that allows negative bases :
<?php
function npow($base, $exp)
{
    $result = pow(abs($base), $exp);
    if ($exp % 2 !== 0) {
        $result = - ($result);
    }
    return $result;
}
?>

janklopper .AT. gmail dot.com (10-Nov-2004 02:26)

since pow doesn't support decimal powers, you can use a different sollution,

thanks to dOt for doing the math!

a^b = e^(b log a)
which is no the 10log but the e-log (aka "ln")

so instead of: pow( $a , 0.6 ) use something like: exp( 0.6 * log($a) )

matthew underscore kay at ml1 dot net (18-Mar-2004 07:03)

As of PHP5beta4, pow() with negative bases appears to work correctly and without errors (from a few cursory tests):

pow(-3, 3) = -27
pow(-3, 2) = 9
pow(-5, -1) = -0.2

bishop (18-Jul-2003 04:01)

A couple of points on pow():
1. One of the official examples of pow(2,8) is not pragmatic; use 1 << 8 as it's substantially faster
2. When passing variables to pow(), cast them otherwise you might get warnings on some versions of PHP
3. All the rules of algebra apply: b**(-e) is 1/(b**e), b**(p/q) is the qth root of b**p

So, e.g., sqrt($x) === pow($x, .5); but sqrt() is faster.