Get the user ID in WooCommerce Subscriptions

WC_Subscription is a expanded version of WC_ORDER, thus you can use the same calls as WC_ORDER.

Your code tweaked:

 add_action( 'woocommerce_scheduled_subscription_trial_end', 'registration_trial_expired', 100 ); function registration_trial_expired( $wc_subscription )     {    $order_billing_email = $wc_subscription->get_billing_email();     $User      = get_user_by( 'email', $order_billing_email );             /    $FirstName = $User->first_name;    $LastName  = $User->last_name;    $UserId    = $User->ID;    }

Retrieve the current user object (WP_User).Wrapper of get_currentuserinfo() using the global variable $current_user.

wp_get_current_user();

But it may be deprecated so You can derived from

$userdata = WP_User::get_data_by( $field, $value );

As class WC_Subscription methods are inherited from WC_Abstract_Order and WC_Order classes, you can use get_user_id() method this way:

$userid = $wc_subscription->get_user_id();

This code is tested and works with WC_Subscription instance object

So your code will be:

add_action( 'woocommerce_scheduled_subscription_trial_end', 'registration_trial_expired', 100 );function registration_trial_expired( $wc_subscription ) {    mail("example@gmail.com", "Expired", "Someone's order has expired");    $userid = $wc_subscription->get_user_id(); // <= HERE    mail("example@gmail.com", "Expired", "Someone's order has expired with customer".$userid);    // ...}

Update (on OP's comment)

As the argument $wc_subscription was the subscription ID (and not the Subscription object).

So I have changed the code to:

add_action( 'woocommerce_scheduled_subscription_trial_end', 'registration_trial_expired', 100 );function registration_trial_expired( $subscription_id ) {    // Get an occurrence of the WC_Subscription object    $subscription = wcs_get_subscription( $subscription_id );        // Get the user ID (or customer ID)    $user_id = $subscription->get_user_id();    // The email adress    $email = 'example@gmail.com';    // The theme domain (replace with your theme domain for localisable strings)    $domain = 'woocommerce';        mail( $email, 'Expired', __("Someone’s order has expired", $domain);    mail( $email, 'Expired', __("Someone’s order has expired with customer", $domain) . $user_id );    // ...}