javascript

    $('#send').on('click', function() {
        $.ajax({
            'url': $('#url').val(),
            'type': 'post',
            'complete': function (jqXHR, textStatus) {
                var msg = "Status: " + jqXHR.status + " (" + jqXHR.statusText + " - " + textStatus + ")<br />";
                msg += jqXHR.getAllResponseHeaders().replace(/\n/g, "<br />");

                $('#results').html(msg);
            }
        });
    });

php

    header("HTTP/1.0 200 Some message here");
    flush();
    exit();

Results

Status: 200 (OK - success)
Date: Wed, 07 Dec 2011 21:57:50 GMT 
X-Powered-By: PHP/5.3.6 
Transfer-Encoding: chunked 
Connection: Keep-Alive 
Server: Apache/2.2.17 (Unix) mod_ssl/2.2.17 OpenSSL/0.9.8r DAV/2 PHP/5.3.6 
Content-Type: text/html 
Keep-Alive: timeout=5, max=100 

Question

How do you obtain the "Some message here" area of the header?

http

http protocol

6.1 Status-Line

The very first type of an answer message may be the Status-Line, composed of the protocol version then a number status code and it is connected textual phrase, with every element separated by SP figures. No CR or LF is permitted except within the final CRLF sequence.

   Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

First got it. It's jqXHR.statusText.

$.get("test.php").complete(function(jqXHR) {
    console.log(jqXHR.statusText);
});

Just attempted it in Chrome together with your exact PHP code.

Perhaps you have attempted xhrobject.getResponseHeader() yet? jQuery docs say it is also available there. If you do not be aware of header's title, try getAllResponseHeaders().

Also, are you able to observe that message inside your browser's debugging console (network tab, connection headers)? When not there, it'll hardly be accessible from js.