发送ajax响应的最佳做法是什么？

I have a webapp that calls php thus:

    var jqxhr = 
        $.getJSON( url )
        .done( function( data, status ) {
            if( status == 'success' ) {
                rv = true;
                // initialise the user object
                app.user = new User( data );

                // post login-processing
                app.postLogin();
            } else {
                // login failed
                console.log( "autologin failed!" );
            }
        })
        .fail( function() {
            console.log( "autologin fail" );
    }); // end var jqxhr =

and I'm wondering what is accepted as best practice in the php script?

For example, if all works well, my php returns:

header('Content-type: application/json; charset=utf-8');
print json_encode($row);

If there's an unexpected error, such as bad SQL, my php returns:

header('HTTP/1.1 500 Internal Server Error');
header('Content-Type: application/json; charset=UTF-8');
etc.

prior to calling die(). I then handle this in .fail().

But what if I want to return an EXPECTED error, such as 'Invalid username or password'? Is it ok to use 'Internal Server Error', or should I be using some kind of json response protocol to inform the javascript of the error, and looking for this in .done()?

Any advice much appreciated.

Thanks

Mini

I think you should check the header response from server-side script and use switch case accordingly. For eg:- if the header response is json then use different case, if header response is failed ajax call then use different case.