Hi there,
looking at your file structure looks like you are using smarty framework? as far as your php code is good, yes all old host are using version 5.6, but once you want to upgrade to version 7.1 code may fail cuz there are some major changes carried out in version 7.1. One of them is we can not declare string variable and later use it as an array in code, it was ok in older version it worked but most of scripts or failing cuz this is no more possible in new php version.
If you looking for serious person who will take the job, work on it start to end and then test every thing for you then message me and also you need to extend your budget as its not and easy job.
Regards,
Ahmed