Uses
| Uses | Description | 
|---|---|
| wp-includes/post.php: get_post_meta() | Retrieves a post meta field for the given post ID. | 
| wp-includes/post.php: get_post() | Retrieves post data given a post ID or post object. | 
Retrieve post thumbnail ID.
(int|false) Post thumbnail ID (which can be 0 if the thumbnail is not set), or false if the post does not exist.
add_theme_support( 'post-thumbnails' ); in its functions.php file. See also Post Thumbnails.File: wp-includes/post-thumbnail-template.php
function get_post_thumbnail_id( $post = null ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return false;
	}
	return (int) get_post_meta( $post->ID, '_thumbnail_id', true );
}  | Version | Description | 
|---|---|
| 5.5.0 | The return value for a non-existing post was changed to false instead of an empty string. | 
| 4.4.0 | $postcan be a post ID or WP_Post object. | 
| 2.9.0 | Introduced. | 
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Licensed under the GNU GPLv2+ License.
    https://developer.wordpress.org/reference/functions/get_post_thumbnail_id