Mysqli execute()执行成功，但没有获取记录

我正在使用mysqli执行select查询…

/** ------------  queries ----------      **/
        $stmt = $mysqli->prepare("SELECT * FROM dept");
        if(! $stmt)
        {
            echo "statement not prepared well";
        }
        else
        {
            echo $mysqli->error;
        }
        if (!$stmt->execute()) {
         echo "Execute failed: (" . $stmt->errno . ") " . $stmt->error;
    }
 // add else
 else{
  echo "Query is successfully executed but no result fetch";
   }
        if (!($res = $stmt->get_result())) {
            echo "Getting result set failed: (" . $stmt->errno . ") " . $stmt->error;
        }
        /** ------------------------------- **/
        #------result ----
        var_dump($res->fetch_all());
        #---------(/result)----

我的问题是execute()工作正常，但无法获取记录…这个表中有大量的数据。它显示"查询已成功执行，但未获取结果"，之后是Fatal error: Call to undefined method mysqli_stmt::get_result()

if (!($res = $stmt->get_result())) {

$res = $stmt->get_result();
if( !$res ) {

$res = $stmt->get_result();
if( $res->num_rows == 0 ) {

$stmt = $pdo->prepare("SELECT * FROM dept");
$stmt->execute();
var_dump($stmt->fetchall());