i have these codes :

foreach ( $ids as $idss ) {
echo  get_post_meta($idss, 'IMDB', true);
}

i have search box with Form element that search and find all id ( $ids ) of searched word .

i want to print all $ids ‘s IMDB field !
i can do it on page.PHP of theme !
but i cant do it on header.PHP

i used global $post; and it didnot worked !

Updates ( answer ) :
i add this line to my code :

require_once( dirname(__DIR__) . '/wp-load.php');