How to troubleshoot 'String could not be parsed as XML' error message in PHP

Problem

When you try to parse XML data in a PHP script, you receive the following error message:

Fatal error: Uncaught exception 'Exception' with message 'String could not be parsed as XML' in FILENAME on line X

Resolution

This error is most likely caused by invalid XML data that you are attempting to handle. Alternatively, the XML data can employ a different character set encoding or contain non-standard characters.

There are several websites that can validate XML data. For example, the website http://www.xmlvalidation.com can check basic XML syntax, as well as validate XML data against external schema, such as DTD (Document Type Definition) documents.