Use of undefined constant

I'm using ajax to send a vote_status URL parameter to a script which updates my db table.

It inserts data but for example if vote_status=up I get this notice.

Use of undefined constant up - assumed 'up'

and if vote_status=down I get

Use of undefined constant up - assumed 'up'

Use of undefined constant down - assumed 'down'

Below is part of the开发者_如何学JAVA script on which I get notices.

$vote_status = $_POST['vote_status'];

        switch ($vote_status) 
        {
        case up:
          $wpdb->query( $wpdb->prepare( "UPDATE wp_posts SET up=up+1 WHERE post_id = $vote_post_id" ) ); 
          echo "Up Added!";
          break;
        case down:
          $wpdb->query( $wpdb->prepare( "UPDATE wp_posts SET down=down+1 WHERE post_id = $vote_post_id" ) ); 
          echo "Down Added!";
          break;
        case block:
          echo "Already Voted!";
          break;
        }

I'm not sure how to fix this. The script looks fine to me..

Enclose the words "up", "down" and "block" in quotes. Currently, PHP is interpreting them as constants.

case "up":
...
case "down":
...
case "block":
...