Fix an existing addon (5 php files) that first worked correctly but after upgrading the main script with their latest version, the addon shows errors. The addon is a payment script that connects to an API of an commercial script.
When trying to buy a product and paying with this addon, these errors are shown:
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/****/public_html/client/includes/[login to view URL] on line 0
Warning: Cannot modify header information - headers already sent by (output started at /home/*******/public_html/client/includes/[login to view URL]) in /home/mundohos/public_html/client/[login to view URL] on line 0
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Please, only respond if you can start immediately and if you are an experienced coder with excellent PHP/MySQL coding skills.