add_action(), add_filter() before or after function.

Question

When looking through Wordpress snippets/tutorials/plugins I often see add_action() and add_filter() being placed before the function is declared:

add_action('publish_post', 'email_friends');

function email_friends($post_ID)  {
   $friends = 'bob@example.org, susie@example.org';
   mail($friends, "sally's blog updated" , 'I just put something on my blog: http://blog.example.com');
   return $post_ID;
}

From a logic standpoint this just doesn't make sense to me. Why would you place the function after it is called in your code? This is usually how I would handle the same situation:

function email_friends($post_ID)  {
   $friends = 'bob@example.org, susie@example.org';
   mail($friends, "sally's blog updated" , 'I just put something on my blog: http://blog.example.com');
   return $post_ID;
}

add_action('publish_post', 'email_friends');

I know both scenarios work, but is there a specific advantage to one or the other? About 90% of the time I see the first scenario being used, so that leads me to believe there is a benefit to this in some way.

Answer 1

It is easier to read: When is what called? If you are debugging a hook you can immediately see if you have to read the function or not: If it is not your hook, you can skip the code.

In my themes and plugins I combine all registrations for actions, filters and shortcodes at the top and I add the hook to the PHPDoc block:

add_action( 'wp_head',  'foo' );
add_action( 'shutdown', 'bar' );

/**
 * Foo you!
 *
 * @wp-hook wp_head
 * @return  void
 */
function foo()
{
    print '<!-- foo -->';
}

Answer 2

There is no real difference actually, I for example prefer to follow first scenario, because it's neater to place calls in one place, and define functions below that. PHP parses the whole document prior to running anything, and if functions are properly defined, everything will work normally, no advantage in either scenario.

I believe the right saying here is: Whatever floats your boat :)