To include PHP file in the WordPress plugin you can add it like below.
include_once('/folder/file.php');or
include( plugin_dir_path( __FILE__ ) . 'folder/file.php');
To include PHP file in the WordPress plugin you can add it like below.
include_once('/folder/file.php');or
include( plugin_dir_path( __FILE__ ) . 'folder/file.php');