php-doc-en/reference/math/functions/mt-srand.xml

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<refentry xmlns="http://docbook.org/ns/docbook" xml:id="function.mt-srand">
 <refnamediv>
  <refname>mt_srand</refname>
  <refpurpose>Seeds the Mersenne Twister Random Number Generator</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>void</type><methodname>mt_srand</methodname>
   <methodparam choice="opt"><type>int</type><parameter>seed</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>mode</parameter><initializer>MT_RAND_MT19937</initializer></methodparam>
  </methodsynopsis>
  <para>
   Seeds the random number generator with
   <parameter>seed</parameter> or with a random value
   if no <parameter>seed</parameter> is given.
  </para>
  
  &note.randomseed;
  
 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>seed</parameter></term>
     <listitem>
      <para>
       An arbitrary <type>int</type> seed value.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>mode</parameter></term>
     <listitem>
      <para>
       Use one of the following constants to specify the implementation of the algorithm to use.
       <informaltable>
        <tgroup cols="2">
         <thead>
          <row>
           <entry>Constant</entry>
           <entry>&Description;</entry>
          </row>
         </thead>
         <tbody>
          <row>
           <entry><constant>MT_RAND_MT19937</constant></entry>
           <entry>
            Uses the fixed, correct, Mersenne Twister implementation, available as of PHP 7.1.0.
           </entry>
          </row>
          <row>
           <entry><constant>MT_RAND_PHP</constant></entry>
           <entry>
            Uses an incorrect Mersenne Twister implementation which was used as the default up till PHP 7.1.0. 
            This mode is available for backward compatibility.
           </entry>
          </row>
         </tbody>
        </tgroup>
       </informaltable>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>
 
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.void;
  </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>7.1.0</entry>
       <entry>
        <function>srand</function> <link linkend="migration71.incompatible.rand-srand-aliases">has been made</link> an alias of <function>mt_srand</function>.
       </entry>
      </row>
      <row>
       <entry>7.1.0</entry>
       <entry>
        <function>mt_rand</function> <link linkend="migration71.incompatible.fixes-to-mt_rand-algorithm">has been updated</link> to use the fixed, correct, version of
         the Mersenne Twister algorithm. To fall back to the old behaviour, use <function>mt_srand</function> with <constant>MT_RAND_PHP</constant> as the second parameter.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>
 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>mt_srand</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
// seed with microseconds
function make_seed()
{
  list($usec, $sec) = explode(' ', microtime());
  return $sec + $usec * 1000000;
}
mt_srand(make_seed());
$randval = mt_rand();
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>mt_rand</function></member>
    <member><function>mt_getrandmax</function></member>
    <member><function>srand</function></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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