如何将多个json组合成一个？

        $question = $_GET['question'];        $arr = explode("|",$question);        for($i = 0;$i <= 3; $i++){            $rid = $arr[$i];            $sql = "select * from xs_radio where rid = '$rid'";            $result = mysql_query($sql);    		$arr = array();    		while($row = mysql_fetch_array($result, MYSQL_ASSOC)){    			$ar[] = $row;      		}    		$js = json_encode($ar);            echo $js;        }

[{"rid":"1","title":"\u6211\u662f\u8c01","a":"\u5b8b\u5411\u4e1c","b":"\u9b4f\u4eae","c":"\u738b\u514b\u5a07","d":"\u5b8b\u5b50\u6587","answer":"\u5b8b\u5411\u4e1c"}][{"rid":"2","title":"\u6211\u662f\u8c01","a":"\u9b4f\u4eae","b":"\u5218\u5fb7\u534e","c":"\u674e\u5a1c","d":"\u5f20\u626c","answer":"\u9b4f\u4eae"}][{"rid":"3","title":"\u6211\u662f\u8c01","a":"\u9b4f\u4eae","b":"\u738b\u514b\u5a07","c":"\u674e\u56fd\u4f1f","d":"\u738b\u83f2","answer":"\u738b\u514b\u5a07"}][{"rid":"16","title":"\u6211\u662f\u8c01","a":"\u5218\u5f66\u5bbe","b":"\u674e\u56fd\u632f","c":"\u7a0b\u79c0\u82b9","d":"\u738b\u514b\u5a07","answer":"\u738b\u514b\u5a07"}]

[{"rid":"1","title":"\u6211\u662f\u8c01","a":"\u5b8b\u5411\u4e1c","b":"\u9b4f\u4eae","c":"\u738b\u514b\u5a07","d":"\u5b8b\u5b50\u6587","answer":"\u5b8b\u5411\u4e1c"},{"rid":"2","title":"\u6211\u662f\u8c01","a":"\u9b4f\u4eae","b":"\u5218\u5fb7\u534e","c":"\u674e\u5a1c","d":"\u5f20\u626c","answer":"\u9b4f\u4eae"},{"rid":"3","title":"\u6211\u662f\u8c01","a":"\u9b4f\u4eae","b":"\u738b\u514b\u5a07","c":"\u674e\u56fd\u4f1f","d":"\u738b\u83f2","answer":"\u738b\u514b\u5a07"},{"rid":"16","title":"\u6211\u662f\u8c01","a":"\u5218\u5f66\u5bbe","b":"\u674e\u56fd\u632f","c":"\u7a0b\u79c0\u82b9","d":"\u738b\u514b\u5a07","answer":"\u738b\u514b\u5a07"}]