php-doc-en/reference/bc/functions/bcpowmod.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.bcpowmod" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>bcpowmod</refname>
  <refpurpose>Raise an arbitrary precision number to another, reduced by a specified modulus</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>string</type><methodname>bcpowmod</methodname>
   <methodparam><type>string</type><parameter>base</parameter></methodparam>
   <methodparam><type>string</type><parameter>exponent</parameter></methodparam>
   <methodparam><type>string</type><parameter>modulus</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>scale</parameter><initializer>0</initializer></methodparam>
  </methodsynopsis>
  <para>
   Use the fast-exponentiation method to raise 
   <parameter>base</parameter> to the power
   <parameter>exponent</parameter> with respect to the modulus
   <parameter>modulus</parameter>.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>base</parameter></term>
     <listitem>
      <para>
       The base, as an integral string (i.e. the scale has to be zero).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>exponent</parameter></term>
     <listitem>
      <para>
       The exponent, as an non-negative, integral string (i.e. the scale has to be zero).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>modulus</parameter></term>
     <listitem>
      <para>
       The modulus, as an integral string (i.e. the scale has to be zero).
      </para>
     </listitem>
    </varlistentry>
    &bc.scale.description;
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns the result as a string, or &null; if <parameter>modulus</parameter>
   is <literal>0</literal> or <parameter>exponent</parameter> is negative.
  </para>
 </refsect1>
 
 <refsect1 role="notes">
  &reftitle.notes;
  <note>
   <para>
    Because this method uses the modulus operation, numbers which are not
    positive integers may give unexpected results.
   </para>
  </note>
 </refsect1>
 
 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   The following two statements are functionally identical.  The
   <function>bcpowmod</function> version however, executes in
   less time and can accept larger parameters.
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
$a = bcpowmod($x, $y, $mod);

$b = bcmod(bcpow($x, $y), $mod);

// $a and $b are equal to each other.

?>
]]>
    </programlisting>
   </informalexample>
  </para>
 </refsect1>
 
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>bcpow</function></member>
    <member><function>bcmod</function></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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