Old app built w/ PHP 5.6 needs updating to PHP 7.4 - What does it take?

[humptydumpty]

I have an old app that was made more than a decade ago and is currently running with PHP 5.6 and I was wondering what does it take to upgrade it to PHP 7.4. I'm not talking about upgrading the server PHP version but the app itself so that it works with PHP 7.4.

I found this inspector app called PhpStorm and I've loaded some of the PHP files into it and it does show errors & warnings (also typos) but I'm not sure if it truly is an error or just weird code by the original dev. In short, I don't know what I'm doing lol.

For those familiar with PHP, does the process involve replacing deprecated tags or is it more complicated than that? Is there a need to update the database?

I know it's impossible to answer this stuff without seeing the app so please let me know what info you need from me.

What I know about the app:

• works with PHP 5.6
• uses a MySQL database (don't ask me about the version)
• only runs on a subdomain for some odd reason
• uses the smarty PHP template
• the app's owner is a PITA but he's family

Thanks!