WordPress Get the Page ID outside the loop

Use this global $post instead:

global $post;
echo $post->ID;

You can also create a generic function to get the ID of the post, whether its outside or inside the loop (handles both the cases):

<?php

/**
 * @uses WP_Query
 * @uses get_queried_object()
 * @see get_the_ID()
 * @return int
 */
function get_the_post_id() {
  if (in_the_loop()) {
       $post_id = get_the_ID();
  } else {
       global $wp_query;
       $post_id = $wp_query->get_queried_object_id();
         }
  return $post_id;
} ?>

And simply do:

$page_id = get_the_post_id();

If you're using pretty permalinks, get_query_var('page_id') won't work.

Instead, get the queried object ID from the global $wp_query:

// Since 3.1 - recommended!
$page_object = get_queried_object();
$page_id     = get_queried_object_id();


// "Dirty" pre 3.1
global $wp_query;

$page_object = $wp_query->get_queried_object();
$page_id     = $wp_query->get_queried_object_id();