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<refentry xml:id="function.headers-sent" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>headers_sent</refname>
  <refpurpose>Checks if or where headers have been sent</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>headers_sent</methodname>
   <methodparam choice="opt"><type>string</type><parameter role="reference">filename</parameter><initializer>&null;</initializer></methodparam>
   <methodparam choice="opt"><type>int</type><parameter role="reference">line</parameter><initializer>&null;</initializer></methodparam>
  </methodsynopsis>
  <para>
   Checks if or where headers have been sent.
  </para>
  <para>
   You can't add any more header lines using the <function>header</function>
   function once the header block has already been sent. Using this function
   you can at least prevent getting HTTP header related error messages.
   Another option is to use <link linkend="ref.outcontrol">Output Buffering</link>.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>filename</parameter></term>
     <listitem>
      <para>
       If the optional <parameter>filename</parameter> and
       <parameter>line</parameter> parameters are set, 
       <function>headers_sent</function> will put the PHP source file name
       and line number where output started in the <parameter>filename</parameter>
       and <parameter>line</parameter> variables.
      </para>
      <note>
       <para>
        If the output has started before executing the PHP source file (for example due to a startup error),
        the <parameter>filename</parameter> parameter will be set to an empty string.
       </para>
      </note>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>line</parameter></term>
     <listitem>
      <para>
       The line number where the output started.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   <function>headers_sent</function> will return &false; if no HTTP headers
   have already been sent or &true; otherwise.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>Examples using <function>headers_sent</function></title>
    <programlisting role="php">
<![CDATA[
<?php

// If no headers are sent, send one
if (!headers_sent()) {
    header('Location: http://www.example.com/');
    exit;
}

// An example using the optional file and line parameters
// Note that $filename and $linenum are passed in for later use.
// Do not assign them values beforehand.
if (!headers_sent($filename, $linenum)) {
    header('Location: http://www.example.com/');
    exit;

// You would most likely trigger an error here.
} else {

    echo "Headers already sent in $filename on line $linenum\n" .
          "Cannot redirect, for now please click this <a " .
          "href=\"http://www.example.com\">link</a> instead\n";
    exit;
}

?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>
 
 <refsect1 role="notes">
  &reftitle.notes;
  &note.network.header.sapi;
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>ob_start</function></member>
    <member><function>trigger_error</function></member>
    <member><function>headers_list</function></member>
    <member>
     <function>header</function> for a more detailed discussion of the
     matters involved.
    </member>
   </simplelist>
  </para>
 </refsect1>

</refentry>
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