What is the best way to include my database.php file to any other .php files that require a database connection. I know that some ways are more secure then others. I have my database.php in a private folder and just need to know what code would be the best to include it in other .php files. Thanks in advance.

If the file is required, then using require() should be the solution.

require('path/to/php/file.php');

As suggested above.
Otherwise, use

include('path/to/php/file.php');

Methods and their descriptions

PHP Code:

include('file.php'); 


Includes the file. Does not give a fatal error if include fails but may give a warning.

PHP Code:

require('file.php'); 


Requires that the file is used otherwise it will throw a fatal error.

PHP Code:

require_once('file.php'); 


Doesn't require the same file twice in the same script. This is useful if you want to include config files, etc. that have no need to be required again. I use this more than anything personally.