php-doc-en/reference/math/functions/log.xml

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  <refentry id="function.log">
   <refnamediv>
    <refname>log</refname>
    <refpurpose>Natural logarithm</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>float</type><methodname>log</methodname>
      <methodparam><type>float</type><parameter>arg</parameter></methodparam>
      <methodparam choice="opt"><type>float</type><parameter>base</parameter></methodparam>
     </methodsynopsis>
    <para>
     If the optional <parameter>base</parameter> parameter is specified, 
     <function>log</function> returns log<subscript>base</subscript>
     <parameter>arg</parameter>, otherwise <function>log</function> 
     returns the natural logarithm of <parameter>arg</parameter>.
    </para>
    <note>
     <para>
      The <parameter>base</parameter> parameter became available
      with PHP version 4.3.0.
     </para>
     <para>
      As always you can calculate the logarithm in base <literal>b</literal>
      of a number <literal>n</literal>, but using the mathematical identity:
      log<subscript>b</subscript>(n) = log(n)/log(b), where log is the
      neperian (or natural) logarithm.
     </para>
    </note>
    <para>
     See also <function>exp</function>.
    </para>
   </refsect1>
  </refentry>

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