Rendering conditional HTML elements with PHP

I’m trying to learn PHP to solve an Assesment challenge, where I have to build a Product List & Add Product pages. Reading here and there, watching some tutorials I was able to develop this code so far:

Index.php:

<?php include("db.php"); ?>

<?php include("includes/header.php"); ?>


<div class="container p-4">
    <div class="row">
        <div class="col-md-4">
            <div class="card card-body">
                <!--INPUT FORM
                it will contains the form to add new product to the database:
                Fields: SKU / NAME / PRICE / PROD_TYPE / DVD / BOOK / FUR_H / FUR_W / FUR_L -->
                <form action="index.php" method="POST">
                    <div class="form-group">
                        <input type="text" name="sku" class="form-control" placeholder="Enter SKU Code">
                        <hr/>
                        <input type="text" name="name" class="form-control" placeholder="Enter Product Name">
                        <hr/>
                        <input type="text" name="price" class="form-control" placeholder="Enter Price">
                        <hr/>
                        <label>Product Type</label>
                        <select id="prod_type" name="prod_type" class="form-control" >
                            <option value="">Select Product Type</option>
                            <option value="DVD">DVD</option>
                            <option value="BOOK">BOOK</option>
                            <option value="FUR">FURNITURE</option>
                        </select>
                        <!-- <hr/> -->
                        <!--if the select(prod_type) option = DVD, then show the following fields:
                        Fields: DVD_SIZE
                        if the select(prod_type) option = BOOK, then show the following fields:
                        Fields: BOOK_WEIGHT
                        if the select(prod_type) option = FUR, then show the following fields:
                        Fields: FUR_H / FUR_W / FUR_L -->
                        <hr/>
                        <?php if(isset($_POST['prod_type']) && $_POST['prod_type'] == 'DVD'){ ?>
                            <input type="text" name="dvd_size" class="form-control" placeholder="Enter DVD Size">
                        <?php } else if(isset($_POST['prod_type']) && $_POST['prod_type'] == 'BOOK'){ ?>
                            <input type="text" name="book_weight" class="form-control" placeholder="Enter Book Weight">
                        <?php } else if(isset($_POST['prod_type']) && $_POST['prod_type'] == 'FUR'){ ?>
                            <hr/>
                            <input type="text" name="fur_h" class="form-control" placeholder="Enter Furniture Height">
                            <hr/>
                            <input type="text" name="fur_w" class="form-control" placeholder="Enter Furniture Width">
                            <hr/>
                            <input type="text" name="fur_l" class="form-control" placeholder="Enter Furniture Length">
                        <?php } ?>
                        <!-- <hr/>     -->
                    </div>
                    <hr/>
                    <input type="submit" name="add_product" class="btn btn-success w-100" value="Add Product">

                </form>
            </div>
        </div>
        <div class="col-md-8">

        </div>
    </div>
</div>


<?php include("includes/footer.php"); ?>
    

Thing is, inputs for DVD Size, BOOK Weight and Forniture dimensions ( H, W & L ) should be render depending User selection on #prod_type. Currently those inputs show up after the User choose a select option, and hit Add Product button ( related to POST method I pressume, is the closest that i get )

Answer

PHP runs entirely on the server. If you want things to happen in the browser when your user interacts with your webpage (without performing a round-trip to the server), Javascript is your friend.

In this case, I would give each input a data attribute to say which product type they are relevant for. Then you can write a Javascript function to handle changes to the prod_type field and show/hide the correct fields.

For example:

function toggleFields() {
    var productType = document.getElementById('prod_type').value;
    var fields = document.querySelectorAll('[data-if-prod-type]');
    
    fields.forEach(function (field) {
        if (field.dataset.ifProdType === productType) {
            field.style.display = '';
        } else {
            field.style.display = 'none';
        }
    });
}

document.addEventListener('DOMContentLoaded', function() {
    document.getElementById('prod_type').addEventListener('change', toggleFields);
    
    // Run the toggle function when the DOM is ready loading,
    // so that fields that are not relevant to #prod_type's
    // initial value are hidden.
    toggleFields();
});

<select id="prod_type" name="prod_type" class="form-control">
    <option value="">Select Product Type</option>
    <option value="DVD">DVD</option>
    <option value="BOOK">BOOK</option>
    <option value="FUR">FURNITURE</option>
</select>

<input type="text" name="dvd_size" class="form-control" placeholder="Enter DVD Size" data-if-prod-type="DVD">
<input type="text" name="book_weight" class="form-control" placeholder="Enter Book Weight" data-if-prod-type="BOOK">
<div data-if-prod-type="FUR">
    <hr/>
    <input type="text" name="fur_h" class="form-control" placeholder="Enter Furniture Height">
    <hr/>
    <input type="text" name="fur_w" class="form-control" placeholder="Enter Furniture Width">
    <hr/>
    <input type="text" name="fur_l" class="form-control" placeholder="Enter Furniture Length">
</div>