How to add an xslt stylesheet in php using SimpleXmlElement to an existing xml document?

Question

I have this xml document in a file called text.xml

The xml document currently looks like this:

<?xml version="1.0" encoding="UTF-8"?>
......
<rss version="2.0"...>

Right after the xml version declaration, I want to add the line:

<?xml-stylesheet type="text/xml" href="style.xslt"?>

In more detail this is what I want to do:

1. I first want to convert the string contents of the file to an xml document.

   $mystr = file_get_contents(path to my text.xml file);
   $myxml = new SimpleXmlElement($mystr);
   

2. I then want to add an attribute to my XML document to link an XSLT stylesheet.

   $myxml->addAttribute(_____);
   

using the SimpleXmlElement addAttribute method, but the addAttribute method allows for the name, value, and namespace--How would I even use the namespace? Since I have three parameters it makes the most sense for the href to be in the namespace parameter spot.

So I have tried to fill in the blank with 'xml-stylesheet','text/xml','style.xslt' which doesn't work because I am getting the error that "Attribute requires prefix for namespace". How do I get this to work?

Link to php docs: http://php.net/manual/en/simplexmlelement.addattribute.php

Edit: This is different from the other question because I am trying to link an xslt stylesheet to an rss document. Using the "duplicate questions'" solution causes the xslt stylesheet to be linked after the rss version declaration which is not good enough.

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"...>
<?xml-stylesheet type="text/xml" href="test.xslt"?>
....

Instead I want:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xml" href="test.xslt"?>
<rss version="2.0"...>
....