> If its under 50 lines of code for some simple processing
It doesn't matter how much lines you write. It has nothing to do with the number of lines but the functionalities and how many developers review that piece of code. Furthermore more a third party codebase is usually the code you don't have to test. You keep on talking about frameworks like it's a bad thing, but all frameworks are not equal in size nor features.
You still don't need a framework for that. Just use composer to install a bunch of well-tested, feature-rich, but fully independent packages. Start your script with
include 'vendor/autoload.php';
Don't forget to understand how/why/what those other packages do.
Including a package based on the contents of its packagist description alone is basically like reading the ingredients for a pizza and then eating a bag of flour.
It's the same with frameworks. "Modern PHP" frameworks usually include a ton of composer packages.
Basically, don't use a framework unless you really do need all the features it gives you. Don't just pick a framework and try to cram your code into it for the sole purpose of using a framework.