php中运用ajax时ResponseText返回undefined

php中使用ajax时ResponseText返回undefined

JScript code

<!--

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-->
var URL, ReturnHtml;
var objXMLHTTP = new GetXmlHTTP();
alert(objXMLHTTP); //FF下创建对象XMLHttpRequest成功
URL = "getdata.php?username="+ document.getElementById("username").value;
objXMLHTTP.open("GET", URL, false);
objXMLHTTP.send(null);
ReturnHtml = objXMLHTTP.ResponseText;
alert(ReturnHtml); //FF下总是返回undefined，十分崩溃!!!

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