Include File Syntax

This is my first time programming with PHP. I have seen include files in the beginning of a php block and wonder is there a specific syntax used in these files and how secure this is vs code in my .php page? Can someone show me a small example of what the include file might look like that would contain variables for $mysql server, $username $password and $db_name. Any comments you may have about security involving files containing these variables and where to store them would be greatly appreciated. Thanks In Advance. Kat