wp-includes/post.php:8115Sets the post thumbnail (featured image) for the given post.
$postint|WP_Post$thumbnail_idintint|boolfunction set_post_thumbnail( $post, $thumbnail_id ) { $post = get_post( $post ); $thumbnail_id = absint( $thumbnail_id ); if ( $post && $thumbnail_id && get_post( $thumbnail_id ) ) { if ( wp_get_attachment_image( $thumbnail_id, 'thumbnail' ) ) { return update_post_meta( $post->ID, '_thumbnail_id', $thumbnail_id ); } else { return delete_post_meta( $post->ID, '_thumbnail_id' ); } } return false;}Introduced in 3.1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/post.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.