Button for adding to wishlist works, Database userFavorites included

productpage.php

@@ -73,10 +73,35 @@ $productId = isset($_GET['id']) ? (int)$_GET['id'] : 0;
     ?>
 
     <?php
-    $alreadyInWishlist = false;
+    $alreadyInWishlist = null;
 
+    if (
+            $_SERVER['REQUEST_METHOD'] === 'POST' &&
+            isset($_POST['add_wishlist']) &&
+            isset($_SESSION['user_id'])
+    ) {
+
+        $userId = (int)$_SESSION['user_id'];
+        $productIdPost = (int)$_POST['product_id'];
+
+        if ($productIdPost > 0 && $alreadyInWishlist == null) {
+
+            $stmtFav = mysqli_prepare(
+                    $conn,
+                    "INSERT IGNORE INTO userFavorites (productID, userID) VALUES (?, ?)"
+            );
+
+            if ($stmtFav) {
+                mysqli_stmt_bind_param($stmtFav, 'ii', $productIdPost, $userId);
+                mysqli_stmt_execute($stmtFav);
+                mysqli_stmt_close($stmtFav);
+                $alreadyInWishlist = true;
+            }
+        }
+    }
 
     ?>
+
     <div class="product-wrapper">
         <!-- LINKER BEREICH – BILD -->
         <div class="product-left">