Here is the modified code. i hope it will work. if not let me know.
if (isset($_GET['slug'])){
function get_slug() {
$slug = 'home';
return $slug;
}
} else {
function get_slug() {
global $wpdb;
$sql = "SELECT slug FROM wp_content WHERE slug='".$_GET['slug']."'";
while ($result = $wpdb->get_results($sql)) {
$slug = $result['slug'];
return $slug;
}
}
}