如何解决这个意想不到的T_OBJECT_OPERATOR？

Question

我正在为我的android studio项目开发一个登录。我在000webhost上有我的user_control。这是代码

<?php
header('Content-type: application/json');
require_once 'connection.php';

class User { 

    private $db;
    private $connection;

    function __construct() { 
        $this->db = new DB_Connection();
        $this->connection = $this->db->get_connection();
    }

    public function does_user_exist($email, $password) { 
        $query = "Select * from users where email = '$email' and password = '$password' ";
        $result = mysql_query($this->connection, $query);
        if(mysqli_num_rows($result) > 0) { 
            $json['success'] = 'Welcome ' .$email;
            echo json_encode($json);
            mysqli_close($this->connection);
        }else { 
            $query = "Insert into users(email, password) values ('$email', '$password')";
            $is_inserted = mysqli_query($this->connection, $query);
            if($is_inserted == 1) { 
                $json['success'] = 'Account created, welcome '+ $email;
            } else { 
                $json['error'] = 'Wrong password';
            }

            echo json_encode($json);
            mysql_close($this->connection);

        }
    }
}

$user = new User();
if(isset($_POST['email'],$_POST['password'])) {
    $email = $_POST['email'];
    $password = $_POST['password'];

    if(!empty($email) && !empty($password)) { 
        $encrypted_password = md5($password);
        $user -> does_user_exist($email, $encrypted_password);
    } else { 
        echo json_encode("You must fill both fields.");

    }
}?>

显示的错误是$ this-＆gt; connection = $ this-＆gt; db-get_connection（）;但是我无法弄明白。我应该看到一个$缺失？我有其他一些文件，但我不认为他们正在影响它。如果需要，我可以发布它们。