A use exemple of this method :

Usefull for generating an XML linked with a XSLT !

<?php

// "Create" the document.
$xml = new DOMDocument( "1.0", "ISO-8859-15" );

//to have indented output, not just a line
$xml->preserveWhiteSpace = false;
$xml->formatOutput = true;

// ------------- Interresting part here ------------

//creating an xslt adding processing line
$xslt = $xml->createProcessingInstruction('xml-stylesheet', 'type="text/xsl" href="base.xsl"');

//adding it to the xml
$xml->appendChild($xslt);

// ----------- / Interresting part here -------------

//adding some elements
$root = $xml->createElement("list");
$node = $xml->createElement("contact", "John Doe");
$root-> appendChild($node);
$xml-> appendChild($root);

//creating the file
$xml-> save("output.xml");

?>

output.xml :

<?xml version="1.0" encoding="ISO-8859-15"?>
<?xml-stylesheet type="text/xsl" href="base.xsl"?> //the line has been created successfully
<list>
  <contact>John Doe</contact>
</list>