Resolving PHP Warnings in Debug Log for WordPress Themes and Plugins

I came across this thread: view.colorlibsupport.com/t/how-to-resolve-php-warning-in-the-debug-log/42867, and I’ve been encountering a similar issue along with a few others.

In my case, I’m also seeing a PHP warning in the debug.log:

PHP Warning: Undefined array key “action” in /var/www/mydomain.com/wp-content/themes/sparkling-child/functions.php on line 62 

Additionally, I’m getting these warnings:

PHP Warning: Undefined array key "user_id" in /var/www/mydomain.com/wp-includes/pluggable.php on line 300  
PHP Warning: Trying to access array offset on value of type null in /var/www/mydomain.com/wp-content/plugins/plugin-name/includes/class.php on line 145

I’ve tried adding checks like isset() and array_key_exists() around the problematic lines, but the warnings still persist. I suspect there might be a deeper issue with how the variables are being passed or initialized.

Could this be related to how WordPress handles globals in certain hooks or custom functions? Or could it be a configuration issue with the server (PHP version, error reporting level, etc.)?

Would love to hear suggestions on how to systematically debug and fix these warnings. If anyone has experience dealing with similar issues, please share your approach!