Unwanted appearing in PHP generated XML - why?

The PHP code is this:-

echo "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n";
echo "<example>\n";
echo "</example>";

The result is this:-

<?xml version="1.0" encoding="utf-8" /?>
<head/><example>
</example>

Why is this < head/ > tag appearing? How can I get rid of it?