Is it possible to insert a comment tag into an xml using simplexml?

I am using SimpleXML to build a document, and wondering whether it is possible to insert comment tag to the document like this:

<root>
  <!-- some comment -->
  <value>
</root>

EDIT:

The comment is somewhere in the middle of the document.

<root>
  <tag1 />
  <!-- some comment -->
  <value />
</root>