PHP - If variable is not set, get post variable

I am trying to pass through a variable to a new name if it is set, or get a POST variable if the first variable is not set.

This is my code but it is not working:

if (isset($randombg)) {
    $background = $randombg;    
} else {  
   $background = $_POST['bg']) };
?>

What is wrong with it, or how can I fix this?