Session ile sepet sistemim var sepet kısmında adeti arttırabiliyorum ama eksiltemiyorum bunu için ne yapmalıyım

kodlarım

<?php
  foreach($_GET    as $k => $v) $_GET[$k]    = strip_tags($v);
  foreach($_POST   as $k => $v) $_POST[$k]   = strip_tags($v);


  if($_POST){
    include 'panel/fonksiyon.php';

    if($_POST['islem'] == 'urun-ekle'){
        if(is_numeric($_POST['urun_id']) AND is_numeric($_POST['adet']) AND is_numeric($_POST['secenek_id'])){
            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['secenek_id'] > 0){
                $query = $db->prepare("SELECT * FROM urun_secenek_alt where id=:id LIMIT 1");
                $alt_secenek = $query->execute(array(":id"=>$_POST['secenek_id']));
                $alt_secenek = $query->fetch(PDO::FETCH_ASSOC);
                if($alt_secenek){
                  if($_POST['adet'] > $alt_secenek['stok']){
                    echo 0;
                  }else{
                    $uniqid = $_POST['urun_id'].'-'.$_POST['secenek_id'];
                    $_SESSION['sepet']['urun_id'][$uniqid]      = $_POST['urun_id'];
                    $_SESSION['sepet']['adet'][$uniqid]         = $_POST['adet'];
                    $_SESSION['sepet']['secenek_id'][$uniqid]   = $_POST['secenek_id'];
                    $_SESSION['sepet']['key'][$uniqid]          = $uniqid;
                    echo 2;
                  }
                }else{
                  echo 1;
                }
              }else{
                  if($_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'];
                    $_SESSION['sepet']['secenek_id'][$uniqid]   = 0;
                    $_SESSION['sepet']['key'][$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);

          $secenek = '';

          $alt_secenek_fiyat = 0;
          if($_SESSION['sepet']['secenek_id'][$key] !=0){
            $alt_secenek = $db->prepare("SELECT * FROM urun_secenek_alt where id=:id LIMIT 1");
                    $as = $alt_secenek->execute(array(":id"=>$_SESSION['sepet']['secenek_id'][$key]));
                    $as = $alt_secenek->fetch(PDO::FETCH_ASSOC);

                    $ust_secenek = $db->prepare("SELECT * FROM urun_secenek where id=:id LIMIT 1");
                    $us = $ust_secenek->execute(array(":id"=>$as['urun_secenek_id']));
                    $us = $ust_secenek->fetch(PDO::FETCH_ASSOC);

                    $alt_secenek_fiyat = $as['fiyat'];

          }

          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'] + $alt_secenek_fiyat);
        }

        echo '';
      }else{
        echo 'Sepette Ürün Bulunamadı';
      }
    }else if($_POST['islem'] == 'sepet_sil'){
        $uniqid = $_POST['id'];
        unset($_SESSION['sepet']['urun_id'][$uniqid]);
        unset($_SESSION['sepet']['adet'][$uniqid]);
        unset($_SESSION['sepet']['secenek_id'][$uniqid]);
        unset($_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;
    }

  }

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