PHP User management class

Here is my code for a class that is responsible for managing some common users task like registration, login and logout. It implements also a sub class that can be used to check the session of a logged in user. I've tested it and works fine, but I will improve it as soon as I can. Any suggestion on security improvements are appreciated.

<?php
require_once 'config.php';

Interface UserInterface{

  public function createUser(array $args);
  public function loginUser(array $args);
  public function logoutUser();

}


class User Implements UserInterface{

    private $db;

    private $stmt;

    private $email;

    private $username;

    private $password;

    private $id;

    private $sessioncode;

    private $args;


    /*
    * @param $db must be a PDO instance
    *
    */
    public function __construct(\PDO $db){
        $this->db = $db;
    }

    /*
    * @param $args must be a key/value array
    *
    */
    public function createUser(array $args){

        if($this->checkEmail($args['email'])){
            #header("HTTP/1.1 400 Email already exsist");
            #http_response_code(400);
            echo 'Email address already exsist.';
        }
        elseif($this->checkUsername($args['username'])){
            #header("HTTP/1.1 400 Username already exsist");
            #http_response_code(400);
            echo 'Username already exsist.';
        }
        else {
            $this->password = password_hash($args['password'], PASSWORD_BCRYPT);
            $stmt = $this->db->prepare('INSERT INTO _users (email, username, password) VALUES (?, ?, ? )');
              if($stmt->execute(array($args['email'],$args['username'],$this->password))){
                echo 'Account successful created';
            }
        }
    }

    /*
    * @param $args must be a key/value array
    *
    */
    public function loginUser(array $args){

        $stmt = $this->db->prepare('SELECT id,username,password FROM _users WHERE username = ?');
        $stmt->execute(array($args['username']));
        $result = $stmt->fetch(PDO::FETCH_OBJ);
                if(count($result) > 0 && password_verify($args['password'], $result->password)){
                  UserSessionHelper::setSession($result->username, $result->id);
                    #header('HTTP/1.1 200');
                    echo 'Logged in';
                }
                else {
                    echo 'Wrong username or password';
                    #header('HTTP/1.1 400');
                }
    }

    /*
    * This method wehn called will logout an user
    *
    */
    public function logoutUser(){
        UserSessionHelper::unsetSession();
        header('HTTP/1.1 200');
        #header('Location: ');
        #echo 'Logged out';
    }

    /*
    * @param $email is a key part of the $args array;
    * This method will check if a given email is already registered.
    */
    private function checkEmail($email){

        $stmt = $this->db->prepare('SELECT email FROM _users WHERE email = ?');
            $stmt->execute(array($email));
            $result = $stmt->fetch(PDO::FETCH_OBJ);
              if(count($result) > 0){
                return true;
              }
    }

    /*
    * @param $username is a key part of the $args array;
    * This method will check if a given username is already registered.
    */
    private function checkUsername($username){

        $stmt = $this->db->prepare('SELECT username FROM _users WHERE username = ?');
            $stmt->execute(array($username));
            $result = $stmt->fetch(PDO::FETCH_OBJ);
              if(count($result) > 0){
                return true;
              }
    }

}

interface UserSessionHelperInterface{

  public static function unsetSession();
  public static function setSession(string $username, int $user_id);
  public static function validateSessionID(string $session_id, string $session_hash);

}

class UserSessionHelper implements UserSessionHelperInterface{

    private $session_hash;
    private $username;
    private $user_id;

    /*
    * @params $username must be a string, $user_id must be an integer.
    * This method will register the $_SESSION variables when an user login.
    */
    public static function setSession(string $username,int $user_id){

        $_SESSION['session_'] = self::sessionHash();
        $_SESSION['id_'] = $user_id;
        $_SESSION['username_'] = $username;
        return true;
    }

    /*
    * @param
    * This method will remove all $_SESSION data wehn an user logout.
    */
    public static function unsetSession(){

        session_destroy();
        session_unset();
    }

    /*
    * @params $session_id must be a valid string, $session_hash must be a valid string.
    * This method will check for valid session credentials when an user is logged in.
    */
    public static function validateSessionID(string $session_id,string $session_hash){

        $computed_session_hash = hash('sha384', $session_id);

          if(!preg_match('/^[-,a-zA-Z0-9]{1,128}$/', $session_id) > 0){
              #return header('HTTP/1.1 403');
          }
          elseif(!hash_equals($computed_session_hash, $session_hash)){
              #return header('HTTP/1.1 403');
          }
          else{
              return true;
          }
    }

    /*
    * This method is responsable to hash the regenerated session id, then return it
    *
    */
    private function sessionHash(){

        session_regenerate_id();
        $session_hash = hash('sha384', session_id());
        return $session_hash;
    }

}


?>