<form  action="1.php" method="GET" >
        <select id="myIndex" name="getir" class="form-select" aria-label="Default select example">
            <option value="1" <?php echo isset($_GET['getir']) && $_GET['getir'] == 1 ? ' selected ' : '';?>>deneme1</option>
            <option value="2" <?php echo isset($_GET['getir']) && $_GET['getir'] == 2 ? ' selected ' : '';?>>deneme2</option>
            <option value="3" <?php echo isset($_GET['getir']) && $_GET['getir'] == 3 ? ' selected ' : '';?>>deneme3</option>
       </select><br>
 
    <button class="btn btn-primary btn-lg btn-block btn-success" type="submit" >Test Et</button>
</form>