WC Add to cart

add_action( 'admin_post_nopriv_update_cart', 'scichart_update_cart' );
add_action( 'admin_post_update_cart', 'scichart_update_cart' );

function scichart_update_cart() {

  defined( 'WC_ABSPATH' ) || exit;

  /**
   * Load woo dependencies
   * Normally, they are only loaded on the fronted
   */

  include_once WC_ABSPATH . 'includes/wc-cart-functions.php';
  include_once WC_ABSPATH . 'includes/class-wc-cart.php';

  /**
   * Instantiate the WC() object
   */

  if ( is_null( WC()->cart ) ) {
    wc_load_cart();
  }

  /**
   * Clear the cart
   */

  WC()->cart->empty_cart();

  /**
   * Validate user input
   */

  $to_cart_id = false;

  $variation_id = filter_input(INPUT_POST, 'variation_id', FILTER_SANITIZE_NUMBER_INT);

  $product_id = filter_input(INPUT_POST, 'product_id', FILTER_SANITIZE_NUMBER_INT);

  $quantity = filter_input(INPUT_POST, 'quantity', FILTER_SANITIZE_NUMBER_INT) ?: 1;

  $to_cart_id = ( $variation_id ) ?: ( $product_id ) ?: false;

  if ( $to_cart_id ) {
    /**
     * Programatically add new item with quantity 
     * to the basket
     */
    WC()->cart->add_to_cart( $to_cart_id, $quantity );
  }

  /**
   * Header redirect instead of wp_redirect. 
   * This saves as skipping some apply_filters on the native
   * wordpress redirect helper function
   */

  $redirect = wc_get_cart_url();

  header( "Location: $redirect" );
  exit;
}