deactivate-license.php

function wpi_s2_cancellation_notification() {

    if ( !empty( $_GET['yourauthenticationtoken']) && 'yes' === $_GET['yourauthenticationtoken'] ) {
        // in the URL, I have '?yourauthenticationtoken=yes', that's what I'm looking for here.

        // Only process requests for valid users
        if ( !empty( $_GET['user_id'] ) ) {

            $user_id    = (integer) esc_attr( $_GET['user_id'] );
            $user       = new WP_User($user_id);
            $email      = $user->user_email;

            // Load the Software License Manager table
            // and grab the license key for the user
            global $wpdb;
            $license_table = $wpdb->prefix . 'lic_key_tbl';
            $data = $wpdb->get_results(
                $wpdb->prepare(
                    "SELECT license_key
                    FROM $license_table
                    WHERE email LIKE %s",
                    $email
                )
            );
            
            // if a license was found
            if ( !empty( $data) ) {
    
                // I have set the plugin to allow only one license per user, 
                // so the license is always in the first element of the response. 
                // Adapt to your needs if necessary.
                $license = $data[0]->license_key;

                // Generate the request to deactivate the license key
                $api_params = array(
                    'slm_action'    => 'slm_deactivate',
                    'secret_key'    => '58d24fd20c63f5.84017322',
                    'license_key'   => $license
                );

                // Send query to license manager server
                // to deactivate the license
                $response = wp_remote_get(
                    add_query_arg(
                        $api_params,
                        'https://wp.immo'
                    ),
                    array(
                        'timeout'   => 20,
                        'ssl_verify'    => false
                    )
                );

                // Check for error in the response
                if (is_wp_error($response)){
                    echo "Unexpected Error! The query returned with an error.";
                }
                
                // You could analyze the response and communicate
                // with your customer based on the response. 
                // In my case, S2 Member handles all communication.
              
            }

        }

        exit;

    }
}