variation-add-to-cart-button.php

<?php
/**
 * Single variation cart button
 *
 * @see     https://docs.woocommerce.com/document/template-structure/
 * @author  WooThemes
 * @package WooCommerce/Templates
 * @version 3.0.0
 */
if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

global $product;
?>
<div class="woocommerce-variation-add-to-cart variations_button">
    <?php
        /**
         * @since 3.0.0.
         */
        do_action( 'woocommerce_before_add_to_cart_quantity' );

        woocommerce_quantity_input( array(
            'min_value'   => apply_filters( 'woocommerce_quantity_input_min', $product->get_min_purchase_quantity(), $product ),
            'max_value'   => apply_filters( 'woocommerce_quantity_input_max', $product->get_max_purchase_quantity(), $product ),
            'input_value' => isset( $_POST['quantity'] ) ? wc_stock_amount( $_POST['quantity'] ) : $product->get_min_purchase_quantity(),
        ) );

        /**
         * @since 3.0.0.
         */
        do_action( 'woocommerce_after_add_to_cart_quantity' );
    ?>
    <button type="submit" class="single_add_to_cart_button button alt"><?php echo esc_html( $product->single_add_to_cart_text() ); ?></button>
    <input type="hidden" name="add-to-cart" value="<?php echo absint( $product->get_id() ); ?>" />
    <input type="hidden" name="product_id" value="<?php echo absint( $product->get_id() ); ?>" />
    <input type="hidden" name="variation_id" class="variation_id" value="0" />
</div>