How to get Post ID in Wordpress Admin

Question

I'm developing a Wordpress plugin and I need to get the current Post ID
in the Write Post / Write Page editing screen (outside the loop).

I also need to do it before the "admin_print_scripts" hook, since I would like to pass some data to a javascript file.

I can't use:

$id = $_GET['post'];

because the url doesn't include this variable when you are adding a new post or page.

So far I've tried these options but none of them worked:

A) This returns an ID of 0

function myplugin_setup() {
    global $wp_query;
    $id = $wp_query->get_queried_object_id();
    var_dump($id);
}

add_action('admin_init', 'myplugin_setup' );  

B) This returns an ID of null

function myplugin_setup() {
    global $wp_query;
    $id = $wp_query->post->ID;
    var_dump($id);
}

add_action('admin_init', 'myplugin_setup' );

C) This also returns an ID of null

function myplugin_setup() {
    global $post;
    $id = $post->ID;
    var_dump($id);
}

add_action('admin_init', 'myplugin_setup' );

Answer 1

Make sure you call the global $post after the WordPress query. If you add an action to init or admin_init, the query isn't ready so there's nothing you can get out of the global $post variable.

My advice would be to check the action reference from this page: http://codex.wordpress.org/Plugin_API/Action_Reference and choose one that works for you.

For example, I did this:

add_action( 'admin_head', 'check_page_template' );
function check_page_template() {
    global $post;
    if ( 'page-homepage.php' == get_post_meta( $post->ID, '_wp_page_template', true ) ) {
        // The current page has the foobar template assigned
        // do something

    }
}

And I was able to get the page ID in WP admin

Answer 2

Use:

global $post

at the beginning of your function. You should then have access to $post->ID to get the id of the current post. This will work for new and existing posts.