Doctype in XML file

Question

I can't get it done to add a DOCTYPE to my xml sitemap. W3C keeps showing an error that the doctype wasn't found (which is the only error). Further no errors, but whenever I put some doctype at the top of the document it creates a lot of errors, after revalidation. Probably because any doctype I can find is a XHTML or HTML doctype, can't find a XML doctype. Anyone who can help me out here?

Error from w3c:

No DOCTYPE found! Checking XML syntax only.

Builds the header of the xml file:

 $header = '<'.'?'.'xml version="1.0" encoding="UTF-8"?'.'>'."\n";
 $header .= "\t".'<urlset ';
 $header .= 'xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">'."\n";
 return $header;

xml sitemap:

<?xml version="1.0" encoding="UTF-8"?>
    <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
    <loc>http://www.url.com/path</loc>
    <priority>1.0</priority>
    <changefreq>weekly</changefreq>
    <lastmod>2015-06-05</lastmod>
</url>
</urlset>

EDIT:

Screenshot:

Answer 1

Got it done. I added an inline DTD as DOCTYPE which fixed the problem:

<!DOCTYPE urlset [
<!ELEMENT urlset (url)+>
<!ELEMENT url (loc, priority, changefreq, lastmod)>
<!ELEMENT loc (#PCDATA)>
<!ELEMENT priority (#PCDATA)>
<!ELEMENT changefreq (#PCDATA)>
<!ELEMENT lastmod (#PCDATA)>
<!ATTLIST urlset xmlns CDATA #REQUIRED>