Fusion 10 is very very old. It was current when windows XP was the current operating system. Are you still using windows xp?
You cannot honestly expect Fusion 10 to write code that did not exist for 11 years after it was created can you?
You need to make your server's version of PHP match the old code written by your old software.
You can upgrade to the latest version but even that will not write PHP 7 compatible code so you will still be faced with matching the server's PHP version to the code that is generated even by Fusion 2015..
As an alternative you can find some complaint php processing code for your forms and use that instead of the code that Fusion writes. I am sure everyone is talking about forms processing as that is just about all Fusion 10 can do in PHP.
https://codecanyon.net/item/email-wi...l5-php/4298991
As another alternative you can use an external forms processing site to handle your form to email processing and not use PHP at all on your site.
https://www.123contactform.com/