Adding Multiple Rows to Database Upon User Input - php

Question

I am trying to work out how to add multiple rows in an 'Order Details' table when a form is submitted. The user is able to select multiple sizes which when submitted, should generate up to N number of rows selected by the user.
Below is my current code for the View file:
<form action="../Controller/NewProduct.php" method="POST">
<h4 align="center"> PRODUCT <b>DETAILS</b> </h4>
<br/>
<div class="form-group">
<label for="categoryid">Category</label>
<select class="form-control" name="categoryid" id="categoryid" required>
<option value="1">Tshirt</option>
<option value="2">Swim Shorts</option>
</select>
</div>
<br/>
<div class="form-group">
<label for="productid">Product ID</label>
<input name="productid" class="form-control" id="productid" required>
</div>
<br/>
<div class="form-group">
<label for="name">Product Name</label>
<input name="name" class="form-control" id="name" required>
</div>
<br/>
<div class="form-group">
<label for="description">Product Description</label><br/>
<textarea name="description" rows="4" class="description" placeholder="Enter product description here." required></textarea>
</div>
<br/>
<div class="form-group">
<label for="size">Size</label>
<select class="form-control" id="size" name="size" multiple required>
<option value="S">S</option>
<option value="M">M</option>
<option value="L">L</option>
<option value="XL">XL</option>
<option value="XXL">XXL</option>
</select>
</div>
<br/>
<div class="form-group">
<label for="colour">Colour</label>
<select class="form-control" id="colour" name="colour" required>
<option value="Burgundy">Burgundy</option>
<option value="Navy Blue">Navy Blue</option>
<option value="Red">Red</option>
<option value="White">White</option>
<option value="Grey">Grey</option>
<option value="Black">Black</option>
<option value="Yellow">Yellow</option>
<option value="Green">Green</option>
<option value="Khaki">Khaki</option>
<option value="Blue">Blue</option>
</select>
</div>
<br/>
<div class="form-group">
<label for="unitprice">Unit Price</label>
<input name="unitprice" class="form-control" id="unitprice" placeholder="Do not include the £ sign." required>
</div>
<br/>
<div class="form-group">
<label for="unitsinstock">Units In Stock</label>
<input name="unitsinstock" class="form-control" id="unitsinstock" required>
</div>
<br/>
<div class="form-group">
<label for="pimage">Product Image</label>
<input name="pimage" class="form-control" id="pimage" placeholder="Enter Image URL" required>
</div>
<br/>
<div class="form-group">
<label for="discount">Discount</label>
<input name="discount" class="form-control" id="discount">
</div>
<br/>
<button type="submit" name="submit" class="btn"><i class="fa fa-check"></i> Done</button>
</form>
This this then the file which is run when the form is submitted:
if (isset($_POST['submit'])) {
require_once 'DatabaseConnection.php';
$statement = $pdo->prepare("INSERT INTO Product (categoryid, name, description, colour, unitprice, pimage, discount, productid)
VALUES (:categoryid, :name, :description, :colour, :unitprice, :pimage, :discount, :productid)");
$statement->bindParam(':categoryid', $categoryid);
$statement->bindParam(':name', $name);
$statement->bindParam(':description', $description);
$statement->bindParam(':colour', $colour);
$statement->bindParam(':unitprice', $unitprice);
$statement->bindParam(':pimage', $pimage);
$statement->bindParam(':discount', $discount);
$statement->bindParam(':productid', $productid);
$categoryid = $_POST['categoryid'];
$name = $_POST['name'];
$description = $_POST['description'];
$colour = $_POST['colour'];
$unitprice = $_POST['unitprice'];
$pimage = $_POST['pimage'];
$discount = $_POST['discount'];
$statement2 = $pdo->prepare("INSERT INTO ProductDetails (unitsinstock, size, productid) VALUES (:unitsinstock, :size, :productid)");
$statement2->bindParam(':size', $size);
$statement2->bindParam(':unitsinstock', $unitsinstock);
$statement2->bindParam(':productid', $productid);
$size = $_POST['size'];
$unitsinstock = $_POST['unitsinstock'];
$productid = $_POST['productid'];
$statement->execute();
$statement2->execute();
header('location:../View/AdminAllProducts.php');
}
?>
An example of what I am trying to achieve is like the following: https://imgur.com/a/SPr14

Answer 1

Pick up the sizes array and work on it in a loop:
$sizes = $_POST['size[]'];
foreach ( $sizes as $size )
{
// ... your code of statement2 :
statement2 = $pdo->prepare("INSERT INTO ProductDetails (unitsinstock, size, productid) VALUES (:unitsinstock, :size, :productid)");
$statement2->bindParam(':size', $size);
$statement2->bindParam(':unitsinstock', $unitsinstock);
$statement2->bindParam(':productid', $productid);
$unitsinstock = $_POST['unitsinstock'];
$productid = $_POST['productid'];
$statement->execute();
$statement2->execute();
}

Answer 2

You should change the selects that can be multiple select to be an array (ex: size([]) the in PHP you should work the request value as an array.
You can also concatenat the string in the values to be inserted in order to only do one request to the database.