Functions which allow querying WordPress directly. Useful for AJAX callbacks.

wordpress_query_functions.php

add_filter( 'query_vars', 'theme_query_vars' );
/**
 * Adds variables to $wp->query_vars
 *
 * @param array $vars Global WP query vars.
 * @return array Array used in $wp->query_vars.
 */
function theme_query_vars( $vars ){
	$vars[] = 'qa'; // "query_action" - used for switch in theme_parse_request()
	return $vars;
}

add_action( 'parse_request', 'theme_parse_request' );
/**
 * Parses requests to this theme.
 *
 * @param obj $wp Global WordPress query object.
 * @return string Returns headers and string for specified mime_type response.
 */
function theme_parse_request( $wp ){
	if( ! array_key_exists( 'qa', $wp->query_vars ) )
	  return;
	
	$qa = $wp->query_vars['qa'];
	
	switch( $qa ){
	  default:
	    $return['message'] = 'No switch set for ' . $qa . '.';
	    $type = 'application/json';
	  break;
	}
	
	theme_return_action( $return, $type );
}

/**
 * Returns a response as specified by the supplied mime type
 *
 * @param array||string $return Data to be returned.
 * @param string $mime_type Optional. Valid mime type.
 * @param numeric $status HTTP Status code.
 * @return string HTTP response.
 */
function theme_return_action( $return = '', $mime_type = 'application/json', $status = 200 ){
	if( $mime_type == 'application/json' && is_array( $return ) ) $return = json_encode( $return );
	http_response_code( $status );
	header('Content-type: '.$mime_type.'; charset=utf-8');
    header("Expires: Wed, 28 Jul 1997 02:00:00 GMT");
    header("Cache-Control: no-cache, must-revalidate");
    header("Pragma: no-cache");				
    die( $return );	
}

if (!function_exists('http_response_code')) {
/**
 * Returns HTTP response code string or executes HTTP response header
 *
 * @param numeric $code HTTP response code.
 * @return string HTTP response status.
 */    
    function http_response_code($code = NULL) {

        if ($code !== NULL) {

            switch ($code) {
                case 100: $text = 'Continue'; break;
                case 101: $text = 'Switching Protocols'; break;
                case 200: $text = 'OK'; break;
                case 201: $text = 'Created'; break;
                case 202: $text = 'Accepted'; break;
                case 203: $text = 'Non-Authoritative Information'; break;
                case 204: $text = 'No Content'; break;
                case 205: $text = 'Reset Content'; break;
                case 206: $text = 'Partial Content'; break;
                case 300: $text = 'Multiple Choices'; break;
                case 301: $text = 'Moved Permanently'; break;
                case 302: $text = 'Moved Temporarily'; break;
                case 303: $text = 'See Other'; break;
                case 304: $text = 'Not Modified'; break;
                case 305: $text = 'Use Proxy'; break;
                case 400: $text = 'Bad Request'; break;
                case 401: $text = 'Unauthorized'; break;
                case 402: $text = 'Payment Required'; break;
                case 403: $text = 'Forbidden'; break;
                case 404: $text = 'Not Found'; break;
                case 405: $text = 'Method Not Allowed'; break;
                case 406: $text = 'Not Acceptable'; break;
                case 407: $text = 'Proxy Authentication Required'; break;
                case 408: $text = 'Request Time-out'; break;
                case 409: $text = 'Conflict'; break;
                case 410: $text = 'Gone'; break;
                case 411: $text = 'Length Required'; break;
                case 412: $text = 'Precondition Failed'; break;
                case 413: $text = 'Request Entity Too Large'; break;
                case 414: $text = 'Request-URI Too Large'; break;
                case 415: $text = 'Unsupported Media Type'; break;
                case 500: $text = 'Internal Server Error'; break;
                case 501: $text = 'Not Implemented'; break;
                case 502: $text = 'Bad Gateway'; break;
                case 503: $text = 'Service Unavailable'; break;
                case 504: $text = 'Gateway Time-out'; break;
                case 505: $text = 'HTTP Version not supported'; break;
                default:
                    exit('Unknown http status code "' . htmlentities($code) . '"');
                break;
            }

            $protocol = ( isset( $_SERVER['SERVER_PROTOCOL'] ) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0' );

            header( $protocol . ' ' . $code . ' ' . $text );

            $GLOBALS['http_response_code'] = $code;

        } else {

            $code = (isset($GLOBALS['http_response_code']) ? $GLOBALS['http_response_code'] : 200);

        }

        return $code;

    }
}