Php in laravel, what is wrong with my mysql statement?

$res = DB::connection("mysql_business")->table("t_activity")
            ->select("SELECT app_id,title ,created_at,id
                FROM t_activity
                WHERE created_at > ? ORDER BY created_at DESC", [ "2018-04-01 20:11:12"]);

var_dump($res);//print some database.php information  

print many strings like this
object (IlluminateDatabaseQueryBuilder)-sharp68 (22) {
["connection"] = >
object (IlluminateDatabaseMySqlConnection)-sharp58 (16) {

["pdo":protected]=>
object(Closure)-sharp57 (2) {
  ["static"]=>
  array(1) {
    ["config"]=>
    array(12) {
      ["driver"]=>
      string(5) "mysql"
      ["port"]=>
      string(4) ""************************""
      ["database"]=>
      string(14) "************************"
      ["username"]=>
      string(4) "root"
      ["password"]=>
      string(12) ""************************""
      ["charset"]=>
      string(4) "utf8"
      ["collation"]=>
      string(15) ""************************""
      ["prefix"]=>
      string(0) ""
      ["strict"]=>
      bool(false)
      ["engine"]=>
      NULL
      ["name"]=>
      string(14) ""************************""
      ["host"]=>
      string(33) ""************************""
    }

it just print information in database.php, not the answer I want, why??

and below code is it right

$ret = DB::connection("mysql_business")->table("t_activity")->where("created_at", ">", "2018-04-01 20:11:12")
            ->select("app_id", "title", "created_at", "id")->get();
Mar.04,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3619c-2bfcf.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3619c-2bfcf.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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