What should I do if I encounter the following problems during xml-WeChat development? Developed by PHP.

Warning: simplexml_load_string () expectsparameter2tobeaclassnamederivedfromSimpleXMLElement, & #39; SimplexXMLElement & #39; givenintextMsg. phponline28token is successfully verified. some codes are as follows: $ postStr $ GLOBALS [& quot; HTTP_RAW_POST_DATA & quot;]; a method for retrieving POST data xmlphp

Warning: simplexml_load_string () expects parameter 2 to be a class name derived from SimpleXMLElement, 'simplexxmlelement 'given in textMsg. php on line 28

The token is successfully verified. some codes are as follows:
$ PostStr = $ GLOBALS ["HTTP_RAW_POST_DATA"]; // A method for obtaining POST data

// $ PostObj = simplexml_load_string ($ postStr, 'simplexxmlelement ', LIBXML_NOCDATA); $ postObj = simplexml_load_string ($ postStr, 'simplexmlelement', LIBXML_NOCDATA ); // load the XML string into the object and parse xml $ fromUsername = $ postObj-> FromUserName; $ toUsername = $ postObj-> ToUserName; $ keyword = trim ($ postObj-> Content); $ magType-> $ postObj-> MsgId; $ CreateTime = intval ($ postObj-> CreateTime );