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<refentry xml:id="function.hash-init" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>hash_init</refname>
  <refpurpose>Initialize an incremental hashing context</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>HashContext</type><methodname>hash_init</methodname>
   <methodparam><type>string</type><parameter>algo</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>flags</parameter><initializer>0</initializer></methodparam>
   <methodparam choice="opt"><type>string</type><parameter>key</parameter><initializer>""</initializer></methodparam>
   <methodparam choice="opt"><type>array</type><parameter>options</parameter><initializer>[]</initializer></methodparam>
  </methodsynopsis>

 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>algo</parameter></term>
     <listitem>
      <para>
       Name of selected hashing algorithm (i.e. "md5", "sha256", "haval160,4", etc..). For a list of supported algorithms see <function>hash_algos</function>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>flags</parameter></term>
     <listitem>
      <para>
       Optional settings for hash generation, currently supports only one option:
       <constant>HASH_HMAC</constant>.  When specified, the <parameter>key</parameter>
       <emphasis>must</emphasis> be specified.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>key</parameter></term>
     <listitem>
      <para>
       When <constant>HASH_HMAC</constant> is specified for <parameter>flags</parameter>,
       a shared secret key to be used with the HMAC hashing method must be supplied in this
       parameter.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>options</parameter></term>
     <listitem>
      <para>
       An array of options for the various hashing algorithms.  Currently, only the "seed" parameter is
       supported by the MurmurHash variants.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns a Hashing Context for use with <function>hash_update</function>,
   <function>hash_update_stream</function>, <function>hash_update_file</function>,
   and <function>hash_final</function>.
  </para>
 </refsect1>

 <refsect1 role="errors">
  &reftitle.errors;
  <para>
   Throws a <classname>ValueError</classname> exception if
   <parameter>algo</parameter> is unknown or is a non-cryptographic hash
   function, or if <parameter>key</parameter> is empty.
  </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>8.1.0</entry>
       <entry>The <parameter>options</parameter> parameter has been added.</entry>
      </row>
      <row>
       <entry>8.0.0</entry>
       <entry>
        Now throws an <classname>ValueError</classname> exception if the
        <parameter>algo</parameter> is unknown or is a non-cryptographic hash
        function, or if <parameter>key</parameter> is empty. Previously,
        &false; was returned and an <constant>E_WARNING</constant> message was
        emitted.
       </entry>
      </row>
      <row>
       <entry>7.2.0</entry>
       <entry>Usage of non-cryptographic hash functions (adler32, crc32, crc32b, fnv132, fnv1a32, fnv164, fnv1a64, joaat) with <constant>HASH_HMAC</constant> was disabled.</entry>
      </row>
      <row>
       <entry>7.2.0</entry>
       <entry>
        Return <classname>HashContext</classname> instead of resource.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>Incremental hashing example</title>
    <programlisting role="php">
<![CDATA[
<?php
$hash = hash('sha256', 'The quick brown fox jumped over the lazy dog.');

$ctx = hash_init('sha256');
hash_update($ctx, 'The quick brown fox ');
hash_update($ctx, 'jumped over the lazy dog.');
$incremental_hash = hash_final($ctx);

echo $incremental_hash, PHP_EOL;
var_dump($hash === $incremental_hash);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
68b1282b91de2c054c36629cb8dd447f12f096d3e3c587978dc2248444633483
bool(true)
]]>
    </screen>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>hash</function></member>
    <member><function>hash_algos</function></member>
    <member><function>hash_file</function></member>
    <member><function>hash_hmac</function></member>
    <member><function>hash_hmac_file</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>
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