php-doc-en/language/types/resource.xml

<?xml version="1.0" encoding="utf-8"?>
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<sect1 xml:id="language.types.resource">
 <title>Resources</title>
  
 <para>
  A <type>resource</type> is a special variable, holding a reference to an
  external resource. Resources are created and used by special functions. See
  the <link linkend="resource">appendix</link> for a listing of all these
  functions and the corresponding <type>resource</type> types.
 </para>

 <para>
  See also the <function>get_resource_type</function> function.
 </para>

 <sect2 xml:id="language.types.resource.casting">
  <title>Converting to resource</title>
  
  <para>
   As <type>resource</type> variables hold special handlers to opened files,
   database connections, image canvas areas and the like, converting to a
   <type>resource</type> makes no sense.
  </para>
 </sect2>

 <sect2 xml:id="language.types.resource.self-destruct">
  <title>Freeing resources</title>
  
  <para>
   Thanks to the reference-counting system introduced with PHP 4's Zend Engine,
   a <type>resource</type> with no more references to it is detected
   automatically, and it is freed by the garbage collector. For this reason, it
   is rarely necessary to free the memory manually.
  </para>

  <note>
   <simpara>
    Persistent database links are an exception to this rule. They are
    <emphasis>not</emphasis> destroyed by the garbage collector. See the
    <link linkend="features.persistent-connections">persistent
    connections</link> section for more information.
   </simpara>
  </note>
  
 </sect2>
</sect1>
 
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