[wp-hackers] How to make $post->ID global

[Nathaniel Taintor]

> Your include file runs before $post is defined.
>
> The trick to make your include file function is to add it or its code with
> an action that takes place after $post has been defined.
>
> If header.php is okay, use add_action( 'wp_head', 'your_function' ) where
> your function runs the code you want to add to the theme header.php file.
>
>
Or, if you need the post ID before then (but after at least the 'wp' hook),
use the function get_queried_object_id()

 http://queryposts.com/function/get_queried_object_id/