Bootstrap flash alert using Tiga Session

functions.php

class Flash_Alert {

	private $session;

	function __construct() {
		$this->session = new \Tiga\Session();
	}

	public function error($message) {
		$this->add('danger', $message);
	}

	public function success($message) {
		$this->add('success', $message);
	}

	public function info($message) {
		$this->add('info', $message);
	}

	public function warning($message) {
		$this->add('warning', $message);
	}

	public function add($type, $message) {
		$old = $this->session->get('flash_alert_'.$type);
		$this->session->set('flash_alert_'.$type, $old.'<div>'.$message.'</div>');
	}

	public function display() {
		if ($this->session->has('flash_alert_danger')) {
			echo '<div class="alert dismissable alert-danger"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'.$this->session->pull('flash_alert_danger').'</div>';
		}
		if ($this->session->has('flash_alert_info')) {
			echo '<div class="alert dismissable alert-info"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'.$this->session->pull('flash_alert_info').'</div>';
		}
		if ($this->session->has('flash_alert_success')) {
			echo '<div class="alert dismissable alert-success"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'.$this->session->pull('flash_alert_success').'</div>';
		}
		if ($this->session->has('flash_alert_warning')) {
			echo '<div class="alert dismissable alert-danger"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'.$this->session->pull('flash_alert_warning').'</div>';
		}
	}

}

$flash = new Flash_Alert();
$flash->error( "Something happens" );
$flash->display();