A couple of notes on upgrading from PHP 5 to PHP 7.
Firstly you can try to copy your site or code to a server with PHP 7 and see how your PHP 5 application runs and then move forward. You can also try to use a tool to help you with finding code that needs to be upgraded and reading the documentation. See the links below.
https://auth0.com/blog/migrating-a-php5-app-to-7-part-three/