if (isset($_GET['sil'])) {
    $sorgu=$db->prepare("UPDATE `musteri` SET `silinmedurumu` = '1' and 'silenpersonel' ={$_GET['silen_kul']} WHERE musteri_id={$_GET['musteri_id']}");
    $sonuc=$sorgu->execute();

<a href="musteriler.php?sil=sil&musteri_id=<?php echo $musteri['musteri_id'] ?>&silen_kul=<?php echo $_SESSION['kul_isim'] ?>"  onclick="return checkDelete()" class="btn btn-danger"><strong>Sil</strong></a>