To be blunt, most of the tutorials you've found are probably absolute garbage and everything they're telling you is wrong. Few of them actually address serious security concerns, and those that do usually either gloss over it, or fail to address the issues by promoting best practices.
It's not hard, but it can be tricky to do right. You need to keep in mind a bunch of things.
NEVER store passwords, API keys, or other sensitive information in your source code. Use configuration files, especially simple ones in INI, JSON or even XML format. PHP has functions for reading all of these quickly and easily.
NEVER store configuration files in a place that's within your public "web root", that is a place that could be accessed by someone tinkering with the URL to probe for files like these. Even if you have rules in your web server configuration to block these requests those rules might be disabled by accident, a problem that often manifests when you redeploy your application to a new server that isn't configured correctly.
NEVER check your production configuration files into version control. This is how you leak API keys and passwords to would be attackers. For instance, accidentally pushing an Amazon AWS key to a public GitHub repository is often swiftly punished by someone who will use that key to spin up hundreds of expensive instances. It doesn't matter if repositories are private: These can be cloned by people and made public without your knowledge.
ALWAYS do what you can to minimize the number of places where critical passwords, API keys and other credentials are stored. Use a secure storage system like 1Password as a "vault" where the contents are properly and thoroughly encrypted, not something like a Google Doc which could be compromised.
ALWAYS burn all passwords, API keys, and other credentials stored on a server in the event of a compromise. If you don't know how much access had or how long they've had access, re-issue new passwords, generate new API keys, and be absolutely certain the old credentials no longer work. Do not assume you have time to fix this. You might not. Immediately and permanently fix the problem when you realize you've got an issue.
The simplest answer here is to make a config.ini
file with this information in it that's saved outside the web root and kept only on the server. Don't download it. Don't copy it. Don't touch it unless you need to. This helps avoid costly, painful mistakes.