WooCommerce - Basic form for variable product

woocommerce.php

<?php $product = wc_get_product( get_the_ID() ) ?>
<form class="txt" method="post" action="<?php the_permalink() ?>?add=<?php get_the_ID() ?>">
    <span class="ref">Ref. <?php echo $product->get_sku() ?></span>

    <div class="price">
        <p><?php echo woocommerce_price( $product->get_price_including_tax() ) ?></p>
        <p>
    </div>

    <div class="attributes">
        <?php $attributes = $product->get_attributes() // GET ALL ATRIBUTES ?>
        <?php foreach( $attributes as $key => $value ): ?>
            <select name="attribute_pa_<?php echo $attribute_name ?>" id="attribute_pa_<?php echo $attribute_name ?>">
                <?php $attribute_name = wc_get_product_terms( get_the_ID(), $key ) // GET ATTRIBUTE NAME ?>
                <?php $attribute_slug = wc_get_product_terms( get_the_ID(), $key, array( 'fields' => 'slugs' ) ) // GET ATTRIBUTE SLUG ?>
                <?php for ( $i=0; $i<count( $attribute_name ); $i++ ): // array_slice BECAUSE ARRAY INDEX IS NOT SEQUENCIAL ?>
                    <option value="<?php $slug = array_slice( $attribute_slug, $i, 1 ); echo $slug[0]; ?>"><?php $name = array_slice( $attribute_name, $i, 1 ); echo $name[0]; ?></option>
                <?php endfor ?>
            </select>
        <?php endforeach ?>
    </div>

    <input type="hidden" name="add-to-cart" value="<?php echo $product->id ?>">
    <input type="hidden" name="product_id" value="<?php echo $product->id ?>">
    <input type="hidden" name="quantity" value="1">
    <button class="bt-comprar">COMPRAR</button>
</form>