highlight the search term in the title & excerpt in WordPress – insert into functions.php; use <?php search_excerpt_highlight(n); ?> where n is excerpt word limit

search_highlight

function search_excerpt_highlight( $limit ) {
	$excerpt = get_the_excerpt();
	$excerpt = explode(' ', get_the_excerpt(), $limit);

	if ( count( $excerpt ) >= $limit ) {
		array_pop( $excerpt );
		$excerpt = implode(' ', $excerpt);
	} else {
		$excerpt = implode(' ', $excerpt);
	}

	$excerpt = apply_filters( 'get_the_excerpt', $excerpt );

	$allowed_tags = '<p>,<br>,<mark>';
	$excerpt = strip_tags( $excerpt, $allowed_tags );

	$excerpt = preg_replace('`\[[^\]]*\]`', '', $excerpt);

	$keys = implode('|', explode(' ', get_search_query()));
	$excerpt = trim(preg_replace('/(' . $keys .')/iu', '<mark class="search-highlight">\0</mark>', $excerpt));

	echo '<p>' . $excerpt . ' [...]</p>';
}

function search_title_highlight() {
	$title = get_the_title();
	$keys = implode('|', explode(' ', get_search_query()));
	$title = preg_replace('/(' . $keys .')/iu', '<strong class="search-highlight">\0</strong>', $title);

	echo $title;
}