Ajax php xmlHttp.responseXML返回值为null

<?php	header('content-type:text/xml');	echo '<?xml version="1.0" encoding="utf-8" standalone="yes"?>';	echo'<response>';	$name=$_get['name'];	$username=array('wangwei','zhoujianfei','meishibo','quxinglin','wangyuming','liaoguihong','wangchenggao','zhouqian');	if(in_array(strtoupper($name),$username)){		echo 'hello,master'.htmlentities($name).'!';	}else if(trim($name)==''){		echo 'stranger,please tell me your name!';	}else{		echo htmlentities($name).',i don\'t know you!';		}		echo '</response>';?>

		var xmlhttp=createxmlhttprequestobject();				//get xmlhttprequest object		function createxmlhttprequestobject(){			var xmlhttp;			if(window.activexobject){				try{					xmlhttp=new activexobject("microsoft.xmlhttp");					}					catch(e){						xmlhttp=false;						}				}else{					try{						xmlhttp=new xmlhttprequest();												}						catch(e){							xmlhttp=false;							}												}				if(!xmlhttp){								alert("error creating the xmlhttprequest object!");								}else{									return xmlhttp;									}					}							function process(){			if(xmlhttp.readystate==4||xmlhttp.readystate==0){				name=encodeuricomponent(document.getelementbyid("myname").value);				xmlhttp.open("get","quickstart.php?name="+name,true);								xmlhttp.onreadystatechange=handleserverresponse;				xmlhttp.send(null);													}else{					settimeout('process()',1000);					}		}				function handleserverresponse(){			if(xmlhttp.readystate==4){				if(xmlhttp.status==200){					xmlresponse=xmlhttp.responsexml;					alert(xmlhttp.responsexml);					xmldocumentelement=xmlresponse.documentelement;					hellomessage=xmldocumentelement.firstchild.data;					document.getelementbyid('divmessage').innerhtml='<i>'+hellomessage+'</i>';					settimeout('process()',1000);									}else{						alert('there was a problem accessing hte server:'+xmlhttp.statustext);						}				}			}

xmlResponse = xmlHttp.responseXML;if(xmlResponse.xml == '') {  alert(xmlHttp.responseText);  return;}