Hàm mysqli_num_rows() trong PHP - PHP Function

$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 rows in result set
	$rowcount=mysqli_num_rows($result);
	printf("Result set has %d rows.
",$rowcount);
  // Free result set
	mysqli_free_result($result);
}

mysqli_close($con);