Simple Self Closing Shortcode Example

//self enclosing shortcode use as [subtitle class="css-class"]TEXT[/subtitle]
//https://www.hostinger.com/tutorials/create-a-shortcode-in-wordpress

function subtitle_link_att($atts, $content = null) {
	$default = array(
		'class' => '',
	);
	$a = shortcode_atts($default, $atts);
	$content = do_shortcode($content);
	return '<div class="'.($a['class']).'">'.$content.'</div>';
}
add_shortcode('subtitle', 'subtitle_link_att'); //[subtitle class="subtitle"]Link[/subtitle]