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';