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

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<refentry xml:id="function.log" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>log</refname>
  <refpurpose>Natural logarithm</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
   <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>
 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>arg</parameter></term>
     <listitem>
      <para>
       The value to calculate the logarithm for
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>base</parameter></term>
     <listitem>
      <para>
       The optinal logarithmic base to use 
       (defaults to 'e' and so to the natural logarithm).
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   The logarithm of <parameter>arg</parameter> to 
   <parameter>base</parameter>, if given, or the
   natural logarithm.  
  </para>
 </refsect1>
<refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>Since 4.3.0</entry>
       <entry>
        The optinal parameter <parameter>base</parameter> became available.
        For older versions 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.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>log10</function></member>
    <member><function>exp</function></member>
    <member><function>pow</function></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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