PHP Feedback using Session

Feedback.php

<?php
/**
 * Get Feedback
 * Set this on views
 */

class Feedback {
  
  /**
   * renders the feedback messages into the view
   */
  public function renderFeedbackMessages()
  {
      // echo out the feedback messages (errors and success messages etc.),
      // they are in $_SESSION["feedback_positive"] and $_SESSION["feedback_negative"]
      // require VIEWS_PATH . '_templates/feedback.php';
      
      // get the feedback (they are arrays, to make multiple positive/negative messages possible)
      $feedback_positive = Session::get('feedback_positive');
      $feedback_negative = Session::get('feedback_negative');
      $feedback_note = Session::get('feedback_note');
      
      // echo out positive messages
      if (isset($feedback_positive)) {
          echo '<div class="alert bg-success alert-dismissible" role="alert"><button class="close" aria-label="close" data-dismiss="alert" type="button"><span aria-hidden="true">x</span></button>';
              echo '<ul class="list-unstyled"><span class="glyphicon glyphicon-ok">&nbsp;</span><strong>SUCCESS</strong><br /><br />';
                  foreach ($feedback_positive as $feedback) {echo '<li>' . $feedback . '</li>';}
              echo '</ul>';
          echo '</div>';
      }
      // echo out negative messages
      else if (isset($feedback_negative)) {
          echo '<div class="alert bg-danger alert-dismissible" role="alert"><button class="close" aria-label="close" data-dismiss="alert" type="button"><span aria-hidden="true">x</span></button>';
              echo '<ul class="list-unstyled"><span class="glyphicon glyphicon-remove">&nbsp;</span><strong>OOPS</strong><br /><br />';
                  foreach ($feedback_negative as $feedback) {echo '<li>' . $feedback . '</li>';}
              echo '</ul>';
          echo '</div>';
      }
      else if (isset($feedback_note)) {
          echo '<div class="alert bg-success alert-dismissible" role="alert"><button class="close" aria-label="close" data-dismiss="alert" type="button"><span aria-hidden="true">x</span></button>';
              echo '<ul class="list-unstyled"><span class="glyphicon glyphicon-exclamation-sign">&nbsp;</span><strong>ATTENTION</strong><br /><br />';
                  foreach ($feedback_note as $feedback) {echo '<li>' . $feedback . '</li>';}
              echo '</ul>';
          echo '</div>';
      }  

      // delete these messages (as they are not needed anymore and we want to avoid to show them twice
      Session::set('feedback_positive', null);
      Session::set('feedback_negative', null);
      Session::set('feedback_note', null);
  }
}

Session.php

<?php

/**
 * Session class
 *
 * handles the session stuff. creates session when no one exists, sets and
 * gets values, and closes the session properly (=logout). Those methods
 * are STATIC, which means you can call them with Session::get(XXX);
 */
class Session
{
    /**
     * starts the session
     */
    public static function init()
    {
        // if no session exist, start the session
        if (session_id() == '') {
            session_start();
        }
    }

    /**
     * sets a specific value to a specific key of the session
     * @param mixed $key
     * @param mixed $value
     */
    public static function set($key, $value)
    {
        $_SESSION[$key] = $value;
    }

    /**
     * gets/returns the value of a specific key of the session
     * @param mixed $key Usually a string, right ?
     * @return mixed
     */
    public static function get($key)
    {
        if (isset($_SESSION[$key])) {
            return $_SESSION[$key];
        }
    }

    /**
     * deletes the session (= logs the user out)
     */
    public static function destroy() {
        //session_unset();
        session_destroy();
        //session_write_close();
    }

}