Once you saved the file all twitter usernames in posts and comments will automatically be linked to their Twitter profiles. Usernames have to be written under the form @username. @link: http://www.wpbeginner.com/wp-tutorials/how-to-automatically-link-twitter-usernames-in-wordpress/

auto_twitter_link.php

<?php
function twtreplace($content) {
  $twtreplace = preg_replace('/([^a-zA-Z0-9-_&])@([0-9a-zA-Z_]+)/',"$1<a href=\"http://twitter.com/$2\" target=\"_blank\" rel=\"nofollow\">@$2</a>",$content);
	return $twtreplace;
}

add_filter('the_content', 'twtreplace');   
add_filter('comment_text', 'twtreplace');