Pimcore\Tool\Authentication::authenticatePlaintext PHP Method

    public static function authenticatePlaintext($username, $password)
    {
        $user = User::getByName($username);
        // user needs to be active, needs a password and an ID (do not allow system user to login, ...)
        if (self::isValidUser($user)) {
            if (self::verifyPassword($user, $password)) {
                return $user;
            }
        }
        return null;
    }

 public function loginAction()
 {
     $user = null;
     try {
         \Pimcore::getEventManager()->trigger("admin.login.login.authenticate", $this, ["username" => $this->getParam("username"), "password" => $this->getParam("password")]);
         $user = $this->getUser();
         if (!$user instanceof User) {
             if ($this->getParam("password")) {
                 $user = Tool\Authentication::authenticatePlaintext($this->getParam("username"), $this->getParam("password"));
                 if (!$user) {
                     throw new \Exception("Invalid username or password");
                 }
             } else {
                 if ($this->getParam("token")) {
                     $user = Tool\Authentication::authenticateToken($this->getParam("username"), $this->getParam("token"));
                     if (!$user) {
                         throw new \Exception("Invalid username or token");
                     }
                     // save the information to session when the user want's to reset the password
                     // this is because otherwise the old password is required => see also PIMCORE-1468
                     if ($this->getParam("reset")) {
                         Tool\Session::useSession(function ($adminSession) {
                             $adminSession->password_reset = true;
                         });
                     }
                 } else {
                     throw new \Exception("Invalid authentication method, must be either password or token");
                 }
             }
         }
     } catch (\Exception $e) {
         //see if module or plugin authenticates user
         \Pimcore::getEventManager()->trigger("admin.login.login.failed", $this, ["username" => $this->getParam("username"), "password" => $this->getParam("password")]);
         $user = $this->getUser();
         if (!$user instanceof User) {
             $this->writeLogFile($this->getParam("username"), $e->getMessage());
             \Logger::info("Login failed: " . $e);
         }
     }
     if ($user instanceof User && $user->getId() && $user->isActive() && $user->getPassword()) {
         Tool\Session::useSession(function ($adminSession) use($user) {
             $adminSession->user = $user;
             Tool\Session::regenerateId();
         });
         if ($this->getParam('deeplink')) {
             $this->redirect('/admin/login/deeplink/?' . $this->getParam('deeplink'));
         } else {
             $this->redirect("/admin/?_dc=" . time());
         }
     } else {
         $this->redirect("/admin/login/?auth_failed=true");
         exit;
     }
 }