In any custom page you can call below three lines to pull up WordPress stack and then use any WordPress functionality

<?
//Imp to include
include(\'wp-load.php\');
define(\'WP_USE_THEMES\', false);
require(\'wp-blog-header.php\');
// check is user is logged - if yes then print its role
if(is_user_logged_in() ) {
$user = wp_get_current_user();
$role = ( array ) $user->roles;
echo \\\"role is \\\".$role[0];
}
?>

Just include wp-load.php file at the top of the custom PHP file

 require_once(\\\"../../../../wp-load.php\\\");

Note: add slashes and dots ../../../../ as per the custom PHP file exists.

You can access any wordpress inbuilt function in any external file by adding wp-load.php file.

require_once(\\\"wp-load.php\\\");

For more details and example, you can visit How to access Wordpress functions in external file