jquery怎么设置请求消息头

$.ajax({
beforeSend :function(xhr){
xhr.setRequestHeader('消息头','值');//设置消息头
}
});

function makeRequest() {
alert("inside makeRequest()");
var settings = {
type: "GET",
url:URL+"?"+REQUEST,
dataType:"xml",
error: function(XHR,textStatus,errorThrown) {
alert ("XHR="+XHR+"\ntextStatus="+textStatus+"\nerrorThrown=" + errorThrown);
},
success: function(data,textStatus) {
$("body").append(data);
},
headers: {
"Access-Control-Allow-Origin":"http://example.edu",
"Access-Control-Allow-Headers":"X-Requested-With"
}
};
$.ajax(settings);
}

PHP判断是否是Ajax请求

isset($_SERVER['HTTP_X_REQUESTED_WITH']) && $_SERVER['HTTP_X_REQUESTED_WITH']==='XMLHttpRequest';