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>Seed the better 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>
   </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 optional seed value
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </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 4.2.0</entry>
       <entry>
        The <parameter>seed</parameter> becomes optional 
        and defaults to a random value if omitted.
       </entry>
      </row>
      <row>
       <entry>Since 5.2.1</entry>
       <entry>
        The Mersenne Twister implementation in PHP now uses a new seeding
        algorithm by Richard Wagner. Identical seeds no longer produce the same
        sequence of values they did in previous versions. This behavior is not
        expected to change again, but it is considered unsafe to rely upon it
        nonetheless.
       </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 (float) $sec + ((float) $usec * 100000);
}
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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