<div class="counter js-counter" data-min="<?= $_SESSION['sepet']['adet'][$key]; ?>" data-adet-guncelle="<?= $_SESSION['sepet']['adet'][$key]; ?>">
   <button class="counter__button js-counter-button js-counter-minus" type="button">
      <svg class="icon icon-minus-circle">
          <use xlink:href="upload/icon.svg#icon-minus-circle"></use>
      </svg>
   </button>
<div class="counter__value js-counter-value"><?= $_SESSION['sepet']['adet'][$key]; ?></div>
    <button class="counter__button js-counter-button js-counter-plus" type="button">
      <svg class="icon icon-plus-circle">
          <use xlink:href="upload/icon.svg#icon-plus-circle"></use>
      </svg>
    </button>
        <input class="js-counter-input js-counter-result js-counter-babies" type="hidden" value="<?= $_SESSION['sepet']['adet'][$key]; ?>" name="adet">
    </div>

 $(document).on('click','[data-adet-guncelle]',function(){
        $.ajax({
              url: "post.php",
              type: "post",
              data: 'islem=adet_guncelle&id='+$(this).attr('data-adet-guncelle'),
              success: function (x) {
               location.reload();
              }
        });
    });

Burası post.php
   if($_POST['islem'] == 'urun-ekle'){
        if(is_numeric($_POST['urun_id']) AND is_numeric($_POST['adet'])){
            if($_POST['adet'] < 1){
              $_POST['adet'] = 1;
            }
            $urunquery = $db->prepare("SELECT * FROM urun where id=:id LIMIT 1");
            $urun = $urunquery->execute(array(":id"=>$_POST['urun_id']));
            $urun = $urunquery->fetch(PDO::FETCH_ASSOC);
            if($urun){
                  if($_POST['adet'] > $urun['stok']){
                    echo 0;
                  }else{
                    $uniqid = $_POST['urun_id'];
                    $_SESSION['sepet']['urun_id'][$uniqid]      = $_POST['urun_id'];
                    $_SESSION['sepet']['adet'][$uniqid]         = $_SESSION['sepet']['adet'][$uniqid]+$_POST['adet'];
                    $_SESSION['sepet']['key'][$uniqid]          = $uniqid;
                    echo 2;
                  }
            }else{
              echo 1;
            }
        }else{
          echo 1;
        }
    }else if($_POST['islem'] == 'listele'){
      if(@count($_SESSION['sepet']['key']) > 0){
        $toplam = 0;
        foreach ($_SESSION['sepet']['key'] as $key) {

          $urunquery = $db->prepare("SELECT * FROM urun where id=:id LIMIT 1");
          $urun = $urunquery->execute(array(":id"=>$_SESSION['sepet']['urun_id'][$key]));
          $urun = $urunquery->fetch(PDO::FETCH_ASSOC);

          $urunimg = $db->prepare("SELECT * FROM urun_img where urun_id=:urun_id LIMIT 1");
          $uimg = $urunimg->execute(array(":urun_id"=>$_SESSION['sepet']['urun_id'][$key]));
          $uimg = $urunimg->fetch(PDO::FETCH_ASSOC);

          echo '<div class="notifications__list">
                  <a class="notifications__item" href="urun/'.$urun['sef'].'">
                   <div class="notifications__ava">
                  <img class="notifications__pic" src="upload/'.$uimg['img'].'">
                  </div>
                  <div class="notifications__details">
                  <div class="notifications__line">
                  <div class="notifications__user">'.$urun['baslik'].'</div>
                  <div class="notifications__time">'.$_SESSION['sepet']['adet'][$key].' Adet</div>
                  </div>
                  <div class="notifications__text"><span class="notifications__project">'.fiyat($urun['fiyat'] + $alt_secenek_fiyat).' TL</span></div>
                  </div>
                  </a>
                </div>';

          $toplam += $_SESSION['sepet']['adet'][$key] * ($urun['fiyat']);
        }

        echo '<a href="index.php">
                  <button class="widget__btn btn btn_purple " style="margin: 5px;" tabindex="0">Alışverişe Dön </button>
                </a>
                <a href="sepet">
                <button class="widget__btn btn btn_purple " tabindex="0">Satın Al</button>
              </a>';
      }else{
        echo '<center><div class="notifications__user">Sepetinizde Ürün Bulunmuyor</div></center>';
      }
    }else if($_POST['islem'] == 'sepet_sil'){
        $uniqid = $_POST['id'];
        unset($_SESSION['sepet']['urun_id'][$uniqid]);
        unset($_SESSION['sepet']['adet'][$uniqid]);
        unset($_SESSION['sepet']['key'][$uniqid]);
        echo 1;
    }else if($_POST['islem'] == 'sepet_sayisi'){
        echo @count($_SESSION['sepet']['key']);
    }else if($_POST['islem'] == 'favori-ekle'){

      $uniqid = $_POST['urun_id'];
      $_SESSION['favori']['urun_id'][$uniqid]      = $_POST['urun_id'];;
      $_SESSION['favori']['key'][$uniqid]          = $uniqid;
      echo 1;
    }else if($_POST['islem'] == 'favori-sil'){
        $uniqid = $_POST['urun_id'];
        unset($_SESSION['favori']['urun_id'][$uniqid]);
        unset($_SESSION['favori']['key'][$uniqid]);
        echo 1;
    }else if ($_POST['islem'] == 'adet-guncelle') {
       $uniqid = $_SESSION['sepet']['adet'];
       $_SESSION['sepet']['adet'][$uniqid]         = $_SESSION['sepet']['adet'][$uniqid]+$_POST['adet'];
    }

  }

 if($_POST['islem'] == 'favori_sayisi'){
    echo @count($_SESSION['favori']['key']);
  }