Json_encode problem under thinkphp5 - Codes Helper

I set up User.php, User.php under the model layer and have a method getUserInfo to query the data, as follows:

$userModel = new UserModel;
dump($userModel->getUser());
dump(json_encode($userModel->getUser()));

the first printed data is as follows

   object(app\index\model\User)-sharp10 (32) {
     ["dateFormat":protected] => string(5) "Y/m/d"
     ["type":protected] => array(1) {
       ["birthday"] => string(9) "timestamp"
     }
     ["insert":protected] => array(1) {
       ["status"] => int(1)
     }
     ["connection":protected] => array(0) {
     }
     ["parent":protected] => NULL
     ["query":protected] => NULL
     ["name":protected] => string(4) "User"
     ["table":protected] => NULL
     ["class":protected] => string(20) "app\index\model\User"
     ["error":protected] => NULL
     ["validate":protected] => NULL
     ["pk":protected] => NULL
     ["field":protected] => array(0) {
     }
     ["readonly":protected] => array(0) {
     }
     ["visible":protected] => array(0) {
     }
     ["hidden":protected] => array(0) {
     }
     ["append":protected] => array(0) {
     }
     ["data":protected] => array(7) {
       ["id"] => int(8)
       ["nickname"] => string(4) "1111"
       ["name"] => string(4) "c"
       ["password"] => string(6) "111111"
       ["create_time"] => int(0)
       ["update_time"] => int(0)
       ["status"] => int(0)
     }
     ["origin":protected] => array(7) {
       ["id"] => int(8)
       ["nickname"] => string(4) "1111"
       ["name"] => string(4) "c"
       ["password"] => string(6) "111111"
       ["create_time"] => int(0)
       ["update_time"] => int(0)
       ["status"] => int(0)
     }
     ["relation":protected] => array(0) {
     }
     ["auto":protected] => array(0) {
     }
     ["update":protected] => array(0) {
     }
     ["autoWriteTimestamp":protected] => bool(true)
     ["createTime":protected] => string(11) "create_time"
     ["updateTime":protected] => string(11) "update_time"
     ["isUpdate":protected] => bool(true)
     ["updateWhere":protected] => array(1) {
       ["id"] => array(2) {
         [0] => string(2) "eq"
         [1] => int(8)
       }
     }
     ["failException":protected] => bool(false)
     ["useGlobalScope":protected] => bool(true)
     ["batchValidate":protected] => bool(false)
     ["resultSetType":protected] => string(5) "array"
     ["relationWrite":protected] => NULL
   }

and call the json_encode method to print json like this

   string(132) "{"id":8,"nickname":"1111","name":"c\u67d0","password":"111111","create_time":"1970\/01\/01","update_time":"1970\/01\/01","status":0}"

I test the value of json_encode myself. If it is an object, it will output the public properties of the object. The test code is as follows:

    class TestClass
   {
       public $foo;
   
       public function __construct($foo) 
       {
           $this->foo = $foo;
       }
   
       public function __toString() {
           return $this->foo;
       }
   }
   
   $class = new TestClass("Hello");
   var_dump(json_encode($class));

print out string (15) "{" foo ":" Hello "}". Why does json_encode under thinkphp print such data