会话变量中未定义的索引

Question

我正在尝试使用php enter image description here通过会话发送特定productId的产品ID 就像当我点击其中一个时我想将特定产品ID传递到我的下一页enter image description here以便我可以显示数据。

<?php
$sql = "SELECT * FROM products ORDER BY RAND() LIMIT 4" ;
$result= mysqli_query($conn,$sql);
while($row = mysqli_fetch_assoc($result)): ?>
<?php                                                   

$productId =$row['productId'];
$subCatId = $row['subCatId'] ;
$_SESSION['productId'] = $productId;?> //I TRIED THIS WAY BUT SINCE IT IS ON LOOP THIS SETS THE SESSION VARIABLE TO THE PRODUCT THAT IS DISPLAYED AT THE LAST
<?php
$sql = "SELECT * FROM pictures WHERE productId = $productId ";
$results= mysqli_query($conn,$sql);
if($rows = mysqli_fetch_assoc($results)): ?>
<?php $raw_url = $rows['pic'];
$urli = "uploads/".$raw_url;
<?php endif ?>

我是PHP新手，对JS也知之甚少。请建议我应该学会解决这个问题。 谢谢！