For security reasons, ZenCart has error reporting turned off by default. See includes/application_top.php for the way it should look like currently ...

And that is the way it should be in a production site.

However, during development and testing custom PHP code, sometimes you need to see what errors are being displayed. You can change the code to ...

But always remember to set it back to original when you are done development and even when you are not currently debugging.