ajax调用php失败怎么办

abc
$.ajax({
type: "GET", // GET方式不会报错
url: "tmpl.html",
datdType: "html",
data: {
"username": "username",
"password": "password"
},
success: function(data) {
$("p").append(data);
}
});

abc
$.ajax({
type: "GET", // 虽然是GET方式，但是data数据是JSON数据的序列化，也会报错
url: "tmpl.html",
datdType: "html",
data: "{%22username%22:%22username%22,%22password%22:%22password%22}",
success: function(data) {
$("p").append(data);
}
});

abc
$.ajax({
type: "POST", // POST方式一定会报错
url: "tmpl.html",
datdType: "html",
data: {
"username": "username",
"password": "password"
},
success: function(data) {
$("p").append(data);
}
});