Projectsend date add issue

in the code block below, I want to add the gecerligun(number 1-9999) plus the date in the belge_tarihi (d-m-Y) column in the tbl_files table. In the current situation, it adds to today’s date as much as the current number.

$statement = $this->dbh->prepare("SELECT gecerli_gun FROM tbl_categories WHERE id = :id");
$statement->bindParam(':id', $kategoriId, PDO::PARAM_INT);
$statement->execute();

$gecerligun = 0;


$row = $statement->fetch() ;
$gecerligun = $row['gecerli_gun'];

if ($gecerliyil > 0){
    $date1=date_create(date("d-m-Y"));
    date_add($date1,date_interval_create_from_date_string($gecerligun . " days"));
    $result_str = $date1->format('d-m-Y');
    $data["expiry_date"] = $result_str;

$date1=date_create(date("d-m-Y"));

I try to change this code but I couldn’t.