function convertResult($rs, $type, $jsonmain="") {
	// receive a recordset and convert it to csv
	// or to json based on "type" parameter.
	$jsonArray = array();
	$csvString = "";
	$csvcolumns = "";
	$count = 0;
	while($r = $rs->fetch_row()) {
		for($k = 0; $k < count($r); $k++) {
			$rs->field_seek($k);
			$finfo = $rs->fetch_field();
			$jsonArray[$count][$finfo->name] = $r[$k];
			$csvString.=",\"".$r[$k]."\"";
			if (!$count) $csvcolumns.=($csvcolumns?",":"").$finfo->name;
		}
		$csvString.="\n";
		$count++;
	}
	$jsondata = "{\"$jsonmain\":".json_encode($jsonArray)."}";
	$csvdata = str_replace("\n,","\n",$csvcolumns."\n".$csvString);
	return ($type=="csv"?$csvdata:$jsondata);
}

http://www.barattalo.it/2010/01/29/10-php-usefull-functions-for-mysqli-improved-stuff/

MySQLi convert a result set to CSV or to JSON

mysql, php, csv, json

PHP