WordPressPlugin_coolcsn-hello-world.php

<?php
/*
Plugin Name: Coolcsn-Hello-World
Plugin URI: http://coolcs.com/
Description: Simple Hello World Plugin
Version: 1.0.0
Author: Stoyan Cheresharov
Author URI: http://coolcsn.com/
License: MIT
*/

// echo 'Hello World!'; Event handler
function hello_world($content)
{
    if (is_single()) {
        $content .= '<h1>Hello World from my plugin!</h1>';
    }
    return $content;
}

// hoocking
add_filter('the_content', 'hello_world');