Frustrated by Magento? Then you’ll love 
Not strictly a Magento 2 thing, but if you have code that looks like this in your extension
namespace FooBazBar; //... if( $node instanceof SimpleXMLElement) {} You’ll need to make sure that SimpleXMLElement is actually used in your current namespace.
namespace FooBazBar; use SimpleXMLElement //... if( $node instanceof SimpleXMLElement) {} Otherwise the instanceof operator will be looking for an instance of FooBazBarSimpleXMLElement, and not SimpleXMLElement.