Source

File: wp-includes/general-template.php

function single_post_title( $prefix = '', $display = true ) {
	$_post = get_queried_object();

	if ( ! isset( $_post->post_title ) ) {
		return;
	}

	/**
	 * Filters the page title for a single post.
	 *
	 * @since 0.71
	 *
	 * @param string  $_post_title The single post page title.
	 * @param WP_Post $_post       The current post.
	 */
	$title = apply_filters( 'single_post_title', $_post->post_title, $_post );
	if ( $display ) {
		echo $prefix . $title;
	} else {
		return $prefix . $title;
	}
}