$b = get_post( $a);
// post_author already contain author ID.
//So you can assign author's ID to $c same as to $d
$c = $d = $b->post_author;
$b = get_post( $a);
// post_author already contain author ID.
//So you can assign author's ID to $c same as to $d
$c = $d = $b->post_author;