请教 angular 下拉联动及传值的问题

问题：

• 第二个下拉框的内容如何出现？
• post 传值是，第一个下拉框的 value 是object:1这种形式，后端(php)如何接收？

代码：

<div id="login" ng-app='angular_post_demo' ng-controller='sign_up'>
    <input type="text" size="40" ng-model="email">
    <br>
    <input type="password" size="40" ng-model="password">
    <br>
    <select ng-options="universit.uni for universit in arr" ng-model="university"></select>
    <br>
    <select ng-options="university.stu for university in arr" ng-model="student"></select>
    <br>
    <button ng-click="check_credentials()">Login</button>
    <br>
    <span id="message"></span>
</div>

    var app = angular.module('angular_post_demo', []);
    app.controller('sign_up', function($scope, $http) {
        $scope.arr = [{
            "uni": "TS",
            "stu": ["Tom", "AAA", "BBB"]
        }, {
            "uni": "SDU",
            "stu": ["CCC", "DDD"]
        }, {
            "uni": "SDUT",
            "stu": ["EEE", "FFF"]
        }];
        $scope.university = $scope.arr[0];
        $scope.check_credentials = function() {
            document.getElementById('message').textContent = "";
            var request = $http({
                method: "post",
                url: window.location.href + "res.php",
                data: {
                    email: $scope.email,
                    pass: $scope.password,
                    university: $scope.university
                    // student: $scope.student
                },
                headers: { 'Content-Type': 'application/x-www-form-urlencoded' }
            });
            request.success(function(data) {
                document.getElementById('message').textContent = data;
            });
        }
    });