Data compare

I know it’s probably easy, but it’s very frustrating. Ihave this filter, which works:

function filter_where($where = '') {
    $where .= " AND post_date >= '2012-01-08'";
  return $where;
  }

But i can’t make a simple substitution, like

$weekago='2012-01-08';

 $where .= " AND post_date >= $weekago;"

which is driving me nuts.
I’ve tried

$weekago='2012-01-08';
$weekago="'" . $weekago . "'";

but still doesn’t work.