php check if values in form have been filled

I have a form and php that submits the values, how do I make the php to check if the values are not empty?

$XX = mysqli_real_escape_string($link, $_REQUEST['XX']);
$YY = mysqli_real_escape_string($link, $_REQUEST['YY']);

if(empty($XX) || empty($YY))
{
    echo "You need to fill in XX or YY";
}

and the form is here

<form method="POST" action=""> 
 <label for="XX">XX</label><br>
 <label for="YY">YY</label><br>
 <input type="text" name="XX" id="XX"><br>
 <input type="text" name="YY" id="YY"><br>
  <input class="button" type="submit" value="submit"><br>
 </form>