You can include PHP codes within another using the include , require and require_once commands. The best in your case is include, which can be used like this:
include('arquivo.php');
Remember that the file to be called must be in the same directory as the current file, if it is in another folder, or in a previous folder, you need to specify exactly the path, examples:
include('scripts/arquivo.php'); // Arquivo em uma pasta que está dentro do diretório atual
include('../arquivo.php'); // Arquivo em um diretório antes
To know the difference between the commands you can give a search. Here is a quick reference: link