Add Code or Text in Article

/* Add Code or Text in Article*/
add_filter('the_content', 'add_my_content');

function add_my_content($content) {
    $first = 'FIRST TEXT OR CODE HERE';
    $last = 'LAST TEXT OR CODE HERE';

    if (is_single()) {
        $content = $first.$content.$last;
    }

    return $content;
}