Mine

"INSERT INTO expenditures (`order_number`, `description`, `domain`) VALUES ('$order_number','$description','$domain') ON DUPLICATE KEY UPDATE `description`=$description,`domain`=$domain";

HERE IS SHOW CREATE TABLE EXPENDITURES (above sql was just a snippet to make it easier to read)

CREATE TABLE `expenditures` (\n  `order_number` varchar(16) collate latin1_general_ci NOT NULL,\n  `description` varchar(255) collate latin1_general_ci NOT NULL,\n  `domain` varchar(16) collate latin1_general_ci default NULL,\n  `price` double NOT NULL,\n  `billing_cycle` varchar(16) collate latin1_general_ci NOT NULL,\n  `start_date` date NOT NULL,\n  `due_date` date NOT NULL,\n  PRIMARY KEY  (`order_number`),\n  KEY `domain` (`domain`)\n) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci