php model与json_encode/json_decode

<?php 

class UserModel
{
    var  $user_id = 0;
    var  $user_name = ‘‘;
    var  $user_email = ‘‘;
    var  $user_pwd = ‘‘;
    
    function UserModel()
    {
        
    }
}

?>

$md->user_id = ‘3‘;
$md->user_name = ‘Lee‘;
$md->user_email = "928532756@qq.com";
$md->user_pwd = "";
      
$json = json_encode($md) 

exit($json);   

$md = json_decode($json);

var_dump($md);