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<?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>num</parameter></methodparam>
<methodparam><type>string</type><parameter>exponent</parameter></methodparam>
<methodparam><type>string</type><parameter>modulus</parameter></methodparam>
<methodparam choice="opt"><type class="union"><type>int</type><type>null</type></type><parameter>scale</parameter><initializer>&null;</initializer></methodparam>
</methodsynopsis>
<para>
Use the fast-exponentiation method to raise
<parameter>num</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>num</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.
</para>
</refsect1>
<refsect1 role="errors">
&reftitle.errors;
<para>
This function throws a <exceptionname>ValueError</exceptionname> in the following cases:
<simplelist>
<member><parameter>num</parameter>, <parameter>exponent</parameter> or <parameter>modulus</parameter> is not a well-formed BCMath numeric string</member>
<member><parameter>num</parameter>, <parameter>exponent</parameter> or <parameter>modulus</parameter> has a fractional part</member>
<member><parameter>exponent</parameter> is a negative value</member>
<member><parameter>scale</parameter> is outside the valid range</member>
</simplelist>
</para>
<simpara>
This function throws a <exceptionname>DivisionByZeroError</exceptionname> exception if <parameter>modulus</parameter>
is <literal>0</literal>.
</simpara>
</refsect1>
<refsect1 role="changelog">
&reftitle.changelog;
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>&Version;</entry>
<entry>&Description;</entry>
</row>
</thead>
<tbody>
<row>
<entry>8.0.0</entry>
<entry>
<parameter>scale</parameter> is now nullable.
</entry>
</row>
<row>
<entry>8.0.0</entry>
<entry>
Now throws a <exceptionname>ValueError</exceptionname> instead of returning &false; if <parameter>exponent</parameter> is a negative value.
</entry>
</row>
<row>
<entry>8.0.0</entry>
<entry>
Dividing by <literal>0</literal> now throws a <exceptionname>DivisionByZeroError</exceptionname> exception instead of returning &false;.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</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="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="seealso">
&reftitle.seealso;
<para>
<simplelist>
<member><function>bcpow</function></member>
<member><function>bcmod</function></member>
</simplelist>
</para>
</refsect1>
</refentry>
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