You can integrate your PHP file into WordPress with various ways like you can create a plugins and activate plugins and then you get all your PHP file accessible to WP functions.
It’s bit dirty but in your case if you wish you add require_once(rtrim($_SERVER['DOCUMENT_ROOT'], "https://wordpress.stackexchange.com/") . '/wp-load.php'); on top of the files or if all files are loaded through index.php add the code on top of the file and after that you can include get_header() and in footer you can write get_footer().
In response to your question in chat for not accessing this in local host is, localhost $_SERVER[‘DOCUMENT_ROOT’] returns path to localhost/ and your package is accessible on localhost using http://locahost/yoursite You have to modify for localhost or if you test this on server with subdomain you have to modify code like require_once(rtrim($_SERVER['DOCUMENT_ROOT'], "https://wordpress.stackexchange.com/") . '/yoursite/wp-load.php');