if you have something like

ALTER TABLE `accounts_purchases`
  ADD CONSTRAINT `accounts_purchases_account` FOREIGN KEY (`account_id`) REFERENCES `accounts` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `accounts_purchases_item` FOREIGN KEY (`item_id`) REFERENCES `items` (`id`) ON DELETE SET NULL ON UPDATE CASCADE;

and you are about to update those constraints:
You have to drop them first, by getting all their “symbols”…

here is a “simple” regexp to modify SQL export to DROPs

$result = preg_replace(
'/ADD CONSTRAINT (.*?) FOREIGN KEY \(.*([,;])/m', 
'DROP FOREIGN KEY \1\2', 
$subject);