PHP mysqli_num_fields() Function

<?php
$con=mysqli_connect("localhost","my_user","my_password","my_db");
// Check connection
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$sql="SELECT Lastname,Age FROM Persons ORDER BY Lastname";

if ($result=mysqli_query($con,$sql))
  {
  // Return the number of fields in result set
  $fieldcount=mysqli_num_fields($result);
  printf("Result set has %d fields.\n",$fieldcount);
  // Free result set
  mysqli_free_result($result);
  }

mysqli_close($con);
?>