$.ajax({
                    url: 'ajax.php',
                    type: 'POST',
                    data: {'data': 'hello'},
                })
                .done(function() {
                    console.log("success");
                })
                .fail(function() {
                    console.log("error");
                })
                .always(function() {
                    console.log("complete");
                });

PHP文件

对PHP不熟，这样写却报错了，求解释，Notice: Undefined index: data in D:\WorkSpace\demo\php\ajax.php on line 2