Q:

SimpleXML insert Processing Instruction (Stylesheet)

I want to integrate an XSL file in an XML string gived me by php CURL command. 
I tryed this
$output = XML gived me by curl option;
$hotel = simplexml_load_string($output);
$hotel->addAttribute('?xml-stylesheet type=”text/xsl” href=”css/stile.xsl”?'); 
echo $hotel->asXML();

Doing this when I see the XML on browser, I receive the file without the stylesheet. 
Where is my error? 

A:

A SimpleXMLElement does not allow you by default to create and add a Processing Instruction (PI) to a node.  However the sister library DOMDocument allows this.  You can marry the two by extending from SimpleXMLElement and create a function to provide that feature:
class MySimpleXMLElement extends SimpleXMLElement
{
    public function addProcessingInstruction($target, $data = NULL) {
        $node   = dom_import_simplexml($this);
        $pi     = $node->ownerDocument->createProcessingInstruction($target, $data);
        $result = $node->appendChild($pi);
        return $this;
    }
}

This then is easy to use:
$output = '<hotel/>';
$hotel  = simplexml_load_string($output, 'MySimpleXMLElement');
$hotel->addProcessingInstruction('xml-stylesheet', 'type="text/xsl" href="style.xsl"');
$hotel->asXML('php://output');

Exemplary output (beautified):
<? xml version="1.0"? >
<hotel>
  <? xml-stylesheet type="text/xsl" href="style.xsl"? >
</hotel>

Another way is to insert an XML chunk to a simplexml element: "PHP SimpleXML: insert node at certain position" or "Insert XML into a SimpleXMLElement".