Example of registered callback hook - to the "the_title" event

post.php

add_action( 'the_title', 'add_markup_to_post_title' );
/**
 * Adds the HTML Markup to the Post Title.
 *
 * @since 1.0.0
 *
 * @param string $title
 *
 * @return string
 */
function add_markup_to_post_title( $title ) {
	
	if ( is_singular() ) {
		$heading_tag = 'h1';
	} else {
		$title = sprintf( '<a href="%s" rel="bookmark">%s</a>', get_permalink(), $title );
		$heading_tag = 'h2';
	}

	return sprintf( '<%1$s class="entry-title" itemprop="headline">%2$s</%1$s>', $heading_tag, $title );
}