Question: Most solutions I found are for front-end use. This is for a plugin, so the all activity is in the back-end.

How do I get the page ID that's being currently use(edited) in the admin?

**Note** I'm out of the loop. I just need to get the ID of page(not posts) that I'm currently seeing in the back-end.

Answer: 
You can also use

```
$post_id = $_GET['post'];

```

Or you can use a hook (probably better).

```
function id_WPSE_114111() {
    global $post;
    $id = $post->ID;
    // do something
}

add_action( 'admin_notices', 'id_WPSE_114111' );

```

You will need to add a conditional since this will run on all admin pages, I recommend using `get_current_screen();`

For example to run only on *pages*:

```
function id_WPSE_114111() {

    global $my_admin_page;
    $screen = get_current_screen();

    if ( is_admin() && ($screen->id == 'page') ) {
        global $post;
        $id = $post->ID;
        var_dump($id);
    }
}

add_action( 'admin_notices', 'id_WPSE_114111' );

```
This is probably the "most secure" way of checking what the current post is on the post edit (and add) page in wp admin.

```
function wpse114111_get_current_post_id(): ?WP_Post {
    global $post;

        if (empty($post) && array_key_exists('post', $_GET)) {
            $post = get_post($_GET['post']);
        }

        // Optional: get an empty post object from the post_type
        if (empty($post) && array_key_exists('post_type', $_GET)) {
            $object = new stdClass();
            $object->post_type = $_GET['post_type'];
            return new WP_Post($object);
        }

        if (empty($post)) {
            return null;
        }

        return $post;
}

```