How to test htmlpurifier using php?

Question

First i download htmlpurifier-4.8.0. Then uploaded it to my hosting, this is the code without htmlpurifier:

<?PHP
if(isset($_POST["submit"]))
{
    include("connect.php");
    $dirty_html = mysqli_real_escape_string($db_mysqli,$_POST['xx']);
    echo $dirty_html;
}
?>

<form method="post" action="" ENCTYPE = "multipart/form-data">
<input name="xx" type="text">
<input type="submit" name="submit" value="OK">
</form>

In the input name="xx" I fill in data <script>alert("test");</script>777, and gotten echo 777.

And here's the code with htmlpurifier usage:

<?PHP
if(isset($_POST["submit"]))
{
    include("connect.php");
    require_once 'htmlpurifier-4.8.0/library/HTMLPurifier.auto.php'; 
    $purifier = new HTMLPurifier();
    $dirty_html = mysqli_real_escape_string($db_mysqli,$_POST['xx']);
    $clean_html = $purifier->purify($dirty_html);
    echo $clean_html;
}
?>
<form method="post" action="" ENCTYPE = "multipart/form-data">
<input name="xx" type="text">
<input type="submit" name="submit" value="OK">
</form>

I repeat the same test with the input. Can someone explain the difference, why without htmlpurifier and with htmlpurifier I'm getting the same results?

Answer 1

 $dirty_html = mysqli_real_escape_string($db_mysqli,$_POST['xx']);
 // $dirty_html  has 777 value now 
  $clean_html = $purifier->purify($dirty_html);
 // $clean_html has 777 value because there is nothing to
   purify in 777 which is a valid value

mysqli_real_escape_string already converts the input to 777.