This error comes about as a result of XML input data being improperly encoded. Two data sources I was using had encoded things like "&", "<" and ">" by leaving off the semi-colon. In other words, the ampersand had been encoded as "&" instead of "&". SimpleXMLElement doesn't like this and throws a warning fest.
To fix the problem, I added a line before calling SimpleXMLElement:
$xmldata = preg_replace('/&(amp|lt|gt)([^;])/', '&$1;$2', $xmldata); $obj_xml = new SimpleXMLElement($xmldata);
The preg_replace fixes the encoding problem and adds the ampersand for you. Just a note, this will only fix the encoding for the three characters I specified above ("&", "<" and ">"). If there are others that are causing problems, you'll have to add them to the first argument of preg_replace().
Here is another blog/article that helped me discover the underlying issue.