Get rid of the () and the $ inside the $GET[]
($feature_id = $_GET['$feature_id']);
// Change to
$feature_id = $_GET['feature_id';
Your include will only bring up, "feature.php1878.php" which doesn't exist.
Assuming you have a features folder, it should look like this.
include("/features/".$feature_id.".php");
, which will end up like "/features/878.php"
That will give you a real link to a page in the features folder(in the root directory of the webserver)
If you don't have a features folder, then you can drop the first part, ie make it just
include($feature_id.".php");