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 <?php
 /**
 * WooCommerce Extra Feature
 * --------------------------
 *
 * Send an email each time an order with coupon(s) is completed
 * The email contains coupon(s) used during checkout process
 *
 */ 
function woo_email_order_coupons( $order_id ) {
        $order = new WC_Order( $order_id );
        
        if( $order->get_used_coupons() ) {
        
          $to = 'youremail@yourcompany.com';
	        $subject = 'New Order Completed';
	        $headers = 'From: My Name <youremail@yourcompany.com>' . "\r\n";
	        
	        $message = 'A new order has been completed.\n';
	        $message .= 'Order ID: '.$order_id.'\n';
	        $message .= 'Coupons used:\n';
	        
	        foreach( $order->get_used_coupons() as $coupon) {
		        $message .= $coupon.'\n';
	        }
	        @wp_mail( $to, $subject, $message, $headers );
        }
}
add_action( 'woocommerce_thankyou', 'woo_email_order_coupons' );