PHP to prevent direct access to or viewing or downloading the config. PHP file

How can I prevent PHP configuration files from being downloaded or accessed? For example, the server stores the config. php file, which contains the username and password for accessing the database. How to prevent users from viewing this file? Or is PHP designed to avoid directly viewing the file content? From the security perspective, what protection measures should be taken for this system-level file?
Comprehensive answers from netizens
After investigation, the following common methods are obtained:

1. define an identifier variable in the program

The code is as follows:
Define ('in _ SYS ', TRUE );


2. get this variable in config. php.
The code is as follows:
If (! Defined ('in _ SYS ')){
Exit ('Access prohibited ');
}