You were missing an equal sign in your $sql
string. Also while binding the params you have used wrong placeholders,See below:
$sql = "UPDATE feeds SET status=:statuschosen WHERE idUser=:id AND id=:idfeed";
try {
$db = getConnection();
$stmt = $db->prepare($sql);
$stmt->bindParam(":statuschosen", $post->statuschosen);
$stmt->bindParam(":idfeed", $post->idfeed);
$stmt->bindParam(":id", $id);
$stmt->execute();
$db = null;
echo json_encode($post);
} catch(PDOException $e) {
echo '{"error":{"text":'. $e->getMessage() .'}}';
}