Q:

Selected value from dropdown menu doesn't select when click on button in php

I am selecting data from two table "pizza" and "size".In "pizza" table we have a fields name "pizza_id , pizza_name , pizza_img" in "size" table  "pizza_id , size_id , size , price". When we select the the first pizza and their sizes in "size" table are small , medium and large.It shows in drop-down menu.The problem is that when we click on first size of pizza(small) and click on button it shows that u are selecting last (large) size of pizza.Every time i m selecting any size of pizza, it shows that u are selecting large(size_id:3) size of pizza.So i dont no how to fix that.If anyone can understand my problem than please help me i m badly stuck.Thank u
  $query = "SELECT * FROM pizza";
  $run = mysqli_query($con, $query);

  while($row=mysqli_fetch_array($run)){

        $pizza_id = $row['pizza_id'];
        $pizza_name = $row['pizza_name'];
        $pizza_img = $row['pizza_img'];

  echo "<div class='pizza_details'>
  <img src='admin/img/$pizza_img' width='200px' height='150px'  
  id='profile-image' />
  <div class='desc'>
  <ul>
  <li><span style='color:yellow;'>Name</span> : $pizza_name</li>
  </ul>";

  $q = "SELECT * FROM size where pizza_id='$pizza_id'";
  $r = mysqli_query($con, $q);

  echo "<select name='size'>";

  while ($new=mysqli_fetch_array($r)){

          $size_id = $new['size_id']; 
          $size = $new['size'];
          $price = $new['price'];

   echo"<option value='$size_id'>$size</option>";

   }//End-while loop

   echo"</select>
   <ul><li>
   <a href='pizza.php?add_cart=$pizza_id&size_id=$size_id'>Add to Cart</a>
   </li></ul>
  </div>
  <div class='clear'></div>
  </div>";

  }//End-While Loop

A:

    <form action="" method="get" enctype="multipart/form-data">
    <input type="hidden" name="product_id" value="<?php echo $product_id; ?>">
    <select class="form-control" id="a_set_of_values" name="a_set_of_values" >

    <?php

      $q = "SELECT * FROM size WHERE product_id='$product_id'";
      $r = mysqli_query($con, $q);

      while($rows=mysqli_fetch_array($r)){
            $size_id = $rows['size_id'];
            $size = $rows['size']; 
            $price = $rows['price'];
    ?>

    <option value="<?php echo $size_id;?> & <?php echo $price; ?>"><?php echo $size;?> : Rs <?php echo $price;?></option>

    <?php } ?>

    </select>
    <button class="read_mor_btn"  type="submit" name="add_cart" title="Add to Cart"><i class="fa fa-shopping-cart"></i>Add to Cart</button>

    </form> 

Now get all these values (product_id, size_id, price) from above and used explode function to get multiples values in select option.
    //Pick these values
    if(isset($_POST['add_cart'])){

    $product_id= $_POST['product_id'];

    $explode_fun = $_POST['a_set_of_values'];
    $new_data = explode(' ', $explode_fun);
    $size_id = $new_data[0];
    $price = $new_data[2];    
    }

    ?>      

