Can’t get ID of post that relates to the comment

comment_post hook pass to attached functions 3 parameters. The fourth parameter of add_action() indicates to how many parameters the attached function will be able to access.

add_action( 'comment_post', 'save_chbox_in_db', 10, 3 );

function save_chbox_in_db( $comment_ID, $comment_approved, $commentdata )
{
     $post_id = (int)$commentdata['comment_post_ID'];
     //
     // other code
     //
}