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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html> <head> <title>Natural logarithm</title> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="function.log1p.html">log1p</a></div> <div class="next" style="text-align: right; float: right;"><a href="function.max.html">max</a></div> <div class="up"><a href="ref.math.html">Math Functions</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div><hr /><div id="function.log" class="refentry"> <div class="refnamediv"> <h1 class="refname">log</h1> <p class="verinfo">(PHP 4, PHP 5)</p><p class="refpurpose"><span class="refname">log</span> — <span class="dc-title">Natural logarithm</span></p> </div> <div class="refsect1 description"> <h3 class="title">Description</h3> <div class="methodsynopsis dc-description"> <span class="type">float</span> <span class="methodname"><b><b>log</b></b></span> ( <span class="methodparam"><span class="type">float</span> <tt class="parameter">$arg</tt></span> [, <span class="methodparam"><span class="type">float</span> <tt class="parameter">$base</tt></span> ] )</div> <p class="para rdfs-comment"> If the optional <i><tt class="parameter">base</tt></i> parameter is specified, <b>log()</b> returns log<sub class="subscript">base</sub> <i><tt class="parameter">arg</tt></i>, otherwise <b>log()</b> returns the natural logarithm of <i><tt class="parameter">arg</tt></i>. </p> </div> <div class="refsect1 parameters"> <h3 class="title">Parameters</h3> <p class="para"> <dl> <dt> <span class="term"><i><tt class="parameter">arg</tt></i></span> <dd> <p class="para"> The value to calculate the logarithm for </p> </dd> </dt> <dt> <span class="term"><i><tt class="parameter">base</tt></i></span> <dd> <p class="para"> The optional logarithmic base to use (defaults to 'e' and so to the natural logarithm). </p> </dd> </dt> </dl> </p> </div> <div class="refsect1 returnvalues"> <h3 class="title">Return Values</h3> <p class="para"> The logarithm of <i><tt class="parameter">arg</tt></i> to <i><tt class="parameter">base</tt></i>, if given, or the natural logarithm. </p> </div><div class="refsect1 changelog"> <h3 class="title">ChangeLog</h3> <p class="para"> <table class="informaltable"> <colgroup> <thead valign="middle"> <tr valign="middle"> <th colspan="1">Version</th> <th colspan="1">Description</th> </tr> </thead> <tbody valign="middle" class="tbody"> <tr valign="middle"> <td colspan="1" rowspan="1" align="left">Since 4.3.0</td> <td colspan="1" rowspan="1" align="left"> The optinal parameter <i><tt class="parameter">base</tt></i> became available. For older versions you can calculate the logarithm in base <i>b</i> of a number <i>n</i>, but using the mathematical identity: log<sub class="subscript">b</sub>(n) = log(n)/log(b), where log is the neperian (or natural) logarithm. </td> </tr> </tbody> </colgroup> </table> </p> </div> <div class="refsect1 seealso"> <h3 class="title">See Also</h3> <p class="para"> <ul class="simplelist"> <li class="member"><a href="function.log10.html" class="function" rel="rdfs-seeAlso">log10()</a></li> <li class="member"><a href="function.exp.html" class="function" rel="rdfs-seeAlso">exp()</a></li> <li class="member"><a href="function.pow.html" class="function" rel="rdfs-seeAlso">pow()</a></li> </ul> </p> </div></div><hr /><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="function.log1p.html">log1p</a></div> <div class="next" style="text-align: right; float: right;"><a href="function.max.html">max</a></div> <div class="up"><a href="ref.math.html">Math Functions</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div></body></html>⌨️ 快捷键说明
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