php mysql stock deduction

when user order  a pro duct I prepaire the order like the picture below  but how can i deduct the product from ' product ' table
if some item order more than I have on stock i must let user correct the quantity that they want until it fit my stock that i have

Please suggest php or sqlstate ment

CREATE TABLE IF NOT EXISTS `product` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ProductId` varchar(80) DEFAULT NULL,
  `ProductName` varchar(255) DEFAULT NULL,
  `ProductType` varchar(255) DEFAULT NULL,
  `ProductPrice` varchar(255) DEFAULT NULL,
  `ProductQuantity` varchar(255) DEFAULT NULL,
  `DateTime` datetime DEFAULT NULL,
  `TimeStamp` timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  UNIQUE KEY `ProductId` (`ProductId`),
  KEY `ProductId_2` (`ProductId`),
  KEY `ProductName` (`ProductName`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=7 ;

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