PHP prototype addressing https://stackoverflow.com/questions/45038265/retrieve-user-id-from-wordpress-to-aanother-php-site/45044122

cutsom_appl.php

<?
// Session may not be started on this script, if not init session
if (!session_id()) {
    session_start();
}

// double check, once for the key, again that the result is an int larger than 0 (as WP will follow the AI rule in the DB)
if (array_key_exists('some_custom_var', $_SESSION) and $_SESSION['some_custom_var'] > 0){
    echo 'User is valid with id:'.$_SESSION['some_custom_var'];
}else{
    echo 'User is NOT valid';
}

echo '<pre>';
var_dump($_SESSION);
echo '</pre>';

functions.php

<?
// in the theme's functions.php file

// Session may not be started on this script, if not init session
if (!session_id()) {
    session_start();
}

// explicitly label as false anytime not set (not logged in)
// not doing this can cause leaks or bad evals
if (!array_key_exists('some_custom_var', $_SESSION)){
    $_SESSION['some_custom_var'] = false;
}

// pulled from/original code/info here: https://codex.wordpress.org/Plugin_API/Action_Reference/wp_authenticate
// hooking into the "when user is wuthenticated" part of WP to set our own easy to use session var
function check_custom_authentication ( $username ) {
  global $wpdb;
  if ( ! username_exists( $username ) ) {  // on fail
    return;
  }
  $userinfo = get_user_by( 'login', $username );
  // set the session
  //$_SESSION['some_custom_var'] = $userinfo; // this will add all the user info to the session if you need more info
  $_SESSION['some_custom_var'] = $userinfo->data->ID;  // sets the user ID to the session
}
add_action( 'wp_authenticate' , 'check_custom_authentication' );  // activates the hook

function clear_custom_authentication() {
  // on logout set the custom var to false so it does not leak/persist unwanted.
  $_SESSION['some_custom_var'] = false;
}
add_action('wp_logout', 'clear_custom_authentication');  // activates the hook