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/*** This code has to be used within context, when an order (subscription) is being processed ***/
// $order is instance of WC_Order
$currency = $order->get_order_currency();
// Allow the PayPal settings to be changed on the fly. The filter could replace the default value of 
// "api_username", "api_password" and so on with the currency specific settings
$paypal_settings = apply_filters('wc_subscriptions_get_paypal_settings', self::get_wc_paypal_settings(), $currency);

// After the filter, you can still refer to the original setting names. If no substitution was performed,
// you will get the default ones, as you already do
$api_username = $paypal_settings['api_username'];
$api_password= $paypal_settings['api_password'];
$api_signature= $paypal_settings['api_signature'];