What is the best way to output plugin result in certain url

Question

Knowing that wordpress doesn't follow MVC pattern. What is the best way in worpress to output plugin's result in a certain url. lets say I would like to show "hello world" in main area under "www.example.com/show-hello-world" url. thanks. hope question is clear!

more details:

Let's say url "example.com/show-hello-world"

1. how could I specify template name, let's say "page" ?

Answer 1

There are two steps:

function my_plugin_rewrite_rule() {
  global $wp;

  $wp->add_query_var( 'show_hello_world' );
  add_rewrite_rule( 'show-hello-world/?$', 'index.php?show_hello_world=1', 'top' );
}
add_action( 'init', 'my_plugin_rewrite_rule' );

That takes care of rewriting. Remember to flush the rewrite rules.

Now, your plugin can check for get_query_var( 'show_hello_world' ); and load a certain file:

function my_plugin_template( $path ) {
 if ( get_query_var( 'show_hello_world' ) )
    return locate_template( 'my-plugin.php' );
  else
    return $path;
}
add_filter( 'template_include', 'my_plugin_template' );

Answer 2

In your header.php, page.php, or index.php, add:

<?php
if (is_page('show-hello-world')) { 
   echo "Hello, world!";
}
?>