What is the appropriate HTTP status code response for a general unsuccessful request (not an error)?

Question

I\'m creating a RESTful API that will process a number of user interactions, including placing orders using stored credit cards.

In the case of a successful order, I

Answer 1

I go with 406 Not Acceptable.

Here's a 4xx list:

const HTTP_BAD_REQUEST = 400;
const HTTP_UNAUTHORIZED = 401;
const HTTP_PAYMENT_REQUIRED = 402;
const HTTP_FORBIDDEN = 403;
const HTTP_NOT_FOUND = 404;
const HTTP_METHOD_NOT_ALLOWED = 405;
const HTTP_NOT_ACCEPTABLE = 406;
const HTTP_PROXY_AUTHENTICATION_REQUIRED = 407;
const HTTP_REQUEST_TIMEOUT = 408;
const HTTP_CONFLICT = 409;
const HTTP_GONE = 410;
const HTTP_LENGTH_REQUIRED = 411;
const HTTP_PRECONDITION_FAILED = 412;
const HTTP_REQUEST_ENTITY_TOO_LARGE = 413;
const HTTP_REQUEST_URI_TOO_LONG = 414;
const HTTP_UNSUPPORTED_MEDIA_TYPE = 415;
const HTTP_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
const HTTP_EXPECTATION_FAILED = 417;
const HTTP_I_AM_A_TEAPOT = 418;                                               // RFC2324
const HTTP_MISDIRECTED_REQUEST = 421;                                         // RFC7540
const HTTP_UNPROCESSABLE_ENTITY = 422;                                        // RFC4918
const HTTP_LOCKED = 423;                                                      // RFC4918
const HTTP_FAILED_DEPENDENCY = 424;                                           // RFC4918
const HTTP_RESERVED_FOR_WEBDAV_ADVANCED_COLLECTIONS_EXPIRED_PROPOSAL = 425;   // RFC2817
const HTTP_UPGRADE_REQUIRED = 426;                                            // RFC2817
const HTTP_PRECONDITION_REQUIRED = 428;                                       // RFC6585
const HTTP_TOO_MANY_REQUESTS = 429;                                           // RFC6585