php没有创建唯一的会话

I am having a problem with my mvc framework, my entire site is using a single session id so when I log in using my pc as soon as i visit the site with another device that device uses that same session , Does anyone know where the problem might be this is the code inside my sessions.php file where sessions are set

public static function set($value,$key)
{    
    $_SESSION[$key] = $value;
}

And below is my login function

   $email = $_POST['email'];
   $password = $_POST['password'];
   $password = md5($password);
   $statement = $this->db->prepare("SELECT * FROM users WHERE email=:email AND password=:password");
   $statement->execute(array(':email'=>$email,':password'=>$password));

   $results = $statement->fetch(PDO::FETCH_ASSOC);
   $count = $statement->rowCount();

   if ($count>0) {
       $id = $results['id'];
       session::init();
       session::set($id,"id");

       echo json_encode(array('status'=>true));
   }

You have...

session::set($id,"id");

And your function, once more..

public static function set($value,$key)
{    
    $_SESSION[$key] = $value;
}

You're setting the key of "id" to whatever the "id" is of the record in the database. There's no reason for this value to change, unless you change it in the database.