how to create shoes sizes like puma and adidas website back end

I create the front end but I cannot click it. The sizes are coming from the database in the form of a string then I convert it into the array after that I display it. how to store the size in a PHP variable if I click any size.

      $ids=$_GET['id'];
    
      $_SESSION['ID']=$ids;

  $show="select * from shoes_insertion where shoes_ID={$ids}";

  $showdata=mysqli_query($con,$show);

  $buy=mysqli_fetch_array($showdata);
  ?>
<?php

$str = $buy['size'];

$coma= substr_count($str,",");

for($i=0;$i<=$coma;$i++){

$show=explode(",",$str);
?>
          <div class="col">

<a href="buy.php?ssize=<?php echo $show[$i]; ?>"><h2><?php echo $show[$i];?></h2></a>

In my code, if I click the size it will store ?ssize variable.