php - Auto add to cart a Gift product variation programmatically in WooCommerce?

Solution:

To add a product variation to cart you need the parent variable product id and the product variation Id (both).

Now the code that you are using is obsolete and will not work if customer update cart in cart page, removing the required product id. In this case, the Gift product will be removed after reloading the page or jumping to another page. Sotemplate_redirect is not the right hook to be used.

Try the following instead, that handle simple products and product variations too:

add_action( 'woocommerce_before_calculate_totals', 'wc_auto_add_gift_to_cart' );
function wc_auto_add_gift_to_cart( $cart ) {
    if (is_admin() && !defined('DOING_AJAX'))
        return;
    
    $required_product_id = 37; // The required product Id (or variation Id)
    $parent_gift_id      = 40; // The parent variable product Id (gift) for a product variation (set to zero for simple products)
    $product_gift_id     = 41; // the variation Id or the product Id (gift)
    
    $has_required = $gift_key = false; // Initializing
    
    foreach ( $cart->get_cart() as $cart_item_key => $cart_item ) {
        // Check if required product is in cart
        if( in_array( $required_product_id, array($cart_item['product_id'], $cart_item['variation_id']) ) ) {
           $has_required = true;
        }
        // Check if gifted product is already in cart
        if( $cart_item['data']->get_id() == $product_gift_id ) {
           $gift_key = $cart_item_key;
        }
    }
    
    // If gift is in cart, but not the required product: Remove gift from cart
    if ( ! $has_required && $gift_key ) {
        $cart->remove_cart_item( $gift_key );
    }
    // If gift is not in cart and the required product is in cart: Add gift to cart
    elseif ( $has_required && ! $gift_key ) {
        // For simple products
        if( $parent_gift_id == 0 ) {
            $cart->add_to_cart( $product_gift_id );
        } 
        // For product variations (of a variable product)
        else {
            $cart->add_to_cart( $parent_gift_id, 1, $product_gift_id );
        }
    }
}
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Code goes in functions.php file of your active child theme (or active theme). Tested and works.