if you want create alert css or javascript with single function in CodeIgniter, here the code

alert_helper.php

# TESTED ON CODEIGNITER 3 & 4

// ------------------------------------------------------------------------

if (! function_exists('alert_css'))
{
	/**
	 * Alerts based on Bootstrap
	 *
	 * session()->setFlashdata('item', alert_css('internal server error'));
	 *
	 * @see https://getbootstrap.com/docs/4.3/components/alerts/
	 */
	function alert_css(string $message = '', string $colour = 'danger', string $title = 'notice')
	{
		return '<div class="alert alert-'.strtolower($colour).' alert-dismissible fade show" role="alert"><strong>'.strtoupper($title).'!</strong> '.ucfirst($message).'.<button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button></div>';
	}
}

// ------------------------------------------------------------------------

if (! function_exists('alert_js'))
{
	/**
	 * Alerts based on JavaScript
	 *
	 * session()->setFlashdata('item', alert_js('internal server error'));
	 *
	 * @see https://www.w3schools.com/jsref/met_win_alert.asp
	 */
	function alert_js(string $message = '')
	{
		return '<script language="javascript">alert("'.ucfirst($message).'")</script>';
	}
}