DomDocument :: LoadXML（）：实体'bull'未在Entity中定义

$xml =  '<tag> &bull; </tag>';
$dom = new DOMDocument;
$dom->preserveWhiteSpace = FALSE;
$dom->loadXML($xml);
echo $dom->saveXML();

$xml is utf8 encoded string which is saved in db while exporting it as XML, I am getting the following warning:

DomDocument::LoadXML(): Entity 'bull' not defined in Entity

If using loadHTML instead of loadXML getting following warning:

DomDocument::LoadXML(): Entity 'tag' not defined in Entity