Choosing the right HTTP response code for incorrect POST data

Question

I suspect this is a very trivial question. I'm writing a PHP script to respond to an AJAX query. The query should include some XML data, which the PHP script processes and then returns a response to. There are two error cases I want to consider:

1. No POST data in the request; or
2. Bad data in the XML (either not valid or well-formed XML, or fails some schema checks)

In such cases I believe I should be returning a 4xx response code. Is there anything more appropriate than 400?

More Details

To illustrate the problem further: The client Javascript application is a diagram editor for educational purposes. The user is required to create a diagram that correctly models a given situation. The student can then submit the diagram, whereby an XML serialization of the diagram is POSTed via an AJAX call to the server. A PHP script analyses the diagram XML and constructs an XML report that is sent as the AJAX response to the client. The two situations I originally described (no XML POST data or invalid XML therein) should not happen when requested by the client, but I think it prudent to correctly capture and deal with these situations. Hence my belief that a 4xx response code is appropriate. The XML report structure doesn't cater for these situations, and an empty report would amount to a perfect diagram, which clearly is not appropriate,

Answer 1

Based upon the meanings of the codes in the TCP/IP Guide it seems like 400 is your best choice. Nothing there seems to meet your example.