What is the expected output of ini_set('display_errors', 0)?

@ferdy,
I think that the program flow is:
1. htaccess is called first and decided what to do with URL
2. PHP index.php is called with any relevant URL parameters
3. PHP returns with a http_response(...) code, 200, 404, 500 etc
4. The htaccess script calls the ErrorDocument file if there is a match

This is the relevant CI4 .htaccess file (I'm typing on a tablet and have no access to the CI3 files)

PHP Code:

<IfModule !mod_rewrite.c>
    # If we don't have mod_rewrite installed, all 404's
    # can be sent to index.php, and everything works as normal.
    ErrorDocument 404 index.php
</IfModule> 


Edit:
Check the following link for a detailed explanation of how to handle PHP errors:

https://phpdelusions.net/articles/error_reporting