php的一个小坑，输出不了json_encode

<?phpheader(‘Access-Control-Allow-Origin:*‘); ?header(‘Access-Control-Allow-Methods:GET,POST‘); ?header(‘Access-Control-Allow-Headers:x-requested-with,content-type‘); class Menu { ???// 菜单ID，这些都是从数据库取 ???public $id = 0; ???// 菜单名 ???public $name = ‘‘; ???// 连接地址 ???public $link = ‘‘; ???// 图标 ???public $icon = ‘‘; ???// 子菜单，是一个列表,必须初始化为一个空数组，否则前端会出错 ???public $subMenu = []; ????// 构造函数 ???public function __construct($id, $n, $l, $icon = ‘‘) { ???????$this->id ??= $id; ???????$this->name = $n; ???????$this->link = $l; ???????$this->icon = $icon; ???}}// 菜单项目$userMang = new Menu(‘1000‘, ‘用户管理‘, ‘‘, ‘user‘);$userAdd ?= new Menu(‘1001‘, ‘用户新增‘, ‘/user/add‘);$userList = new Menu(‘1002‘, ‘用户列表‘, ‘/user/list‘);$userMang->subMenu = [$userAdd, $userList];// 站内信管理$msgMang = new Menu(‘1010‘, ‘站内信管理‘, ‘‘, ‘laptop‘);$msgAdd ?= new Menu(‘1011‘, ‘发布站内信‘, ‘/msg/add‘);$msgList = new Menu(‘1012‘, ‘站内信列表‘, ‘/msg/list‘);$msgMang->subMenu = [$msgAdd, $msgList];header(‘Content-type:application/json‘);exit(json_encode([$userMang, $msgMang]));