function sendAsynchronRequest(url,parameter,callback){
createXMLHttpRequest();
if(parameter == null){
xmlHttp.onreadystatechange = callback;
xmlHttp.open("GET",url,true);//当GET请求时,在地址栏中是带参数的,而参数为NULL,所以用get请求,send(null)
xmlHttp.send(null);
}else{
xmlHttp.onreadystatechange = callback;
xmlHttp.open("POST",url,true);//get
xmlHttp.setRequestHeader(
"Content-Type","application/x-www-form-urlencoded;");
xmlHttp.send(parameter);//当POST请求时,在地址栏中是wu参数的,而参数 !=NULL,所以用设置为POST请求,send(parameter)
}
}