Use Wordpress login system on a custom PHP page.

example.php

/**
 * Example of php code using Wordpress' login.
 */
 
require_once '../pages/wp-blog-header.php';

// If the user is not authorized, redirect to the Wordpress login.
auth_redirect();

// Your code here

wp-config.php

// You must set the following values in your wp-config file.

// Your domain name.
define('WP_HOME',    'http://pltrust.dev.krues8dr.com/' );

// Path to wordpress.  Leave this as an empty string if you've installed 
// it in the root.
define('WP_SITEURL', WP_HOME . 'wordpress' );

// The root path.  This needs to be a level that includes both the custom 
// PHP page and Wordpress itself.
define( 'COOKIEPATH', '/' );
define( 'SITECOOKIEPATH', COOKIEPATH );
define('ADMIN_COOKIE_PATH', COOKIEPATH);