Passing a parameter to a theme function

I’ve built my first theme and decided to add a simple post expiry function (since all the plugins that do this are rubbish or cost money) but I’m having trouble passing a paramater.

Here’s the code in the functions.php:

function expirebydays($numberofdays) {
  if $numberofdays > 0 {
    $daylimit = DATE_SUB(NOW(), INTERVAL $numberofdays DAY);
    $dayquery = "UPDATE $wpdb->posts
      SET $wpdb->posts.post_status = 'expired'
      WHERE $wpdb->posts.post_status = 'publish'
      AND $wpdb->posts.post_type = 'post'
      AND $wpdb->posts.post_date < $daylimit"
    $result = $wpdb->get_results($dayquery);
  }
}

Simple enough, but when I upload the new functions file, I get an error:

“Parse error: syntax error, unexpected ‘$numberofdays’ (T_VARIABLE), expecting ‘(‘ in /blahblahblah/functions.php on line xxx”

I don’t see why I get a syntax error when I’m using a passed parameter. Obviously I’m missing something.

(P.S. No, I don’t care about turning it into a proper plugin, having options pages or doing it another, supposedly more efficient way – this is a very simple expiry doodah for my own use! ?? )

Thanks for any help!