$doc = new DOMDocument();
$doc->load('test.xml');
$user = $doc->getElementsByTagName('uname');
$new = $doc->createElement('uname', 'not a test');
$old = $user->item(0);
$test = $doc->replaceChild($new, $old);   <--- LINE 18
echo $doc->saveXML();

Fatal error: Uncaught exception 'DOMException' with message 'Not Found Error' in C:\AppServ\www\EMB\test.php:18 Stack trace: #0 C:\AppServ\www\EMB\test.php(18): DOMNode->replaceChild(Object(DOMElement), Object(DOMElement)) #1 {main} thrown in C:\AppServ\www\EMB\test.php on line 18