使用mysqli计算表中的记录总数

I want to get the total numbers of records in the comments table to store it in a variable $count and display it. what should I add to the following code?

  $stmt = $mysqli->prepare("select count(*) from comments where post_id=?");
  $stmt->bind_param('i',$id); 
  $id = 133;
  $stmt->execute();

what should I add to the following code?

this...

$stmt->bind_result($count);
if ($stmt->fetch()) {
    echo $count;
}

See http://php.net/manual/mysqli-stmt.bind-result.php