What Woocommerce function is called on PayPal IPN response?

Checking the file /wp-content/plugins/woocommerce/classes/gateways/paypal/class-wc-paypal.php, we see that there's an action hook inside the function check_ipn_response:

if ($this->check_ipn_request_is_valid()) :    header('HTTP/1.1 200 OK');    do_action("valid-paypal-standard-ipn-request", $_POST);

You can hook into it like this:

add_action( 'valid-paypal-standard-ipn-request', 'so_12967331_ipn_response', 10, 1 );function so_12967331_ipn_response( $formdata ){    // do your stuff}

Building on @brasofilo's answer, I had to do extra work for each product for the current order.

Note: I'm new to (un)serializing data, so I don't know why I had to unescape the double quotes to get unserialize() to work. It threw an error, otherwise. Maybe there's a better way to handle this.

function so_12967331_ipn_response( $formdata ) {    if ( !empty( $formdata['invoice'] ) && !empty( $formdata['custom'] ) ) {        if( $formdata['payment_status'] == 'Completed' ) {            if( is_serialized( $posted['custom'] ) ) {                // backwards compatible                // unserialize data                $order_data = unserialize( str_replace('\"', '"', $posted['custom'] ) );                $order_id = $order_data[0];            } else {                // custom data was changed to JSON at some point                $order_data = (array)json_decode( $posted['custom'] );                $order_id = $order_data['order_id'];            }            // get order            $order = new WC_Order( $order_id );            // got something to work with?            if ( $order ) {                // get user id                $user_id = get_post_meta( $order_id, '_customer_user', true );                // get user data                $user = get_userdata( $user_id );                // get order items                $items = $order->get_items();                // loop thru each item                foreach( $items as $order_item_id => $item ) {                    $product = new WC_Product( $item['product_id'] );                    // do extra work...                }               }           }    }}