奇怪的XML解析错误

All,

I'm writing PHP on Eclipse and using Wampserver to view my code on Firefox. I'm encountering a weird error. Here is an example of the code that produces the error:

<?php
 header("Content-Type: text/xml; charset=utf-8");
 echo '<?xml version="1.0" encoding="utf-8" ?>';
 $stringResult='ABC';
 echo $stringResult;
?>

This gives me the Firefox yellow screen of death and the following message:

XML Parsing Error: syntax error
Location: http://localhost/Tests/2013.09_xml_parsing_error/
Line Number 1, Column 40:<?xml version="1.0" encoding="utf-8" ?>ABC
----------------------------------------------------------------^

(Note that on Firefox the -----^ points straight after ..."utf-8" ?>)

What's causing this?

EDIT: I'm also getting an error if I just write:

<?php
 header("Content-Type: text/xml; charset=utf-8");
 $stringResult='ABC';
 echo $stringResult;
?>

I get the following error:

XML Parsing Error: syntax error
Location: http://localhost/Tests/2013.09_xml_parsing_error/
Line Number 1, Column 1:ABC
^

You have to send valid xml, which requires a root node, something like

<?php
 header("Content-Type: text/xml; charset=utf-8");
 echo '<?xml version="1.0" encoding="utf-8" ?>';
 $stringResult='<letters>ABC</letters>';
 echo $stringResult;
?>

It points to a wrong xml attribute. Since your string is not a attribute