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Edit File: class-user-integration.php

<?php

defined('ABSPATH') || exit;

/**
 * Class MC4WP_User_Integration
 *
 * @access public
 * @since 2.0
 */
abstract class MC4WP_User_Integration extends MC4WP_Integration
{
    /**
     * @param WP_User $user
     *
     * @return array
     */
    protected function user_merge_vars(WP_User $user)
    {

// start with user_login as name, since that's always known
        $data = [
            'EMAIL' => $user->user_email,
            'NAME'  => $user->user_login,
        ];

if ('' !== $user->first_name) {
            $data['NAME']  = $user->first_name;
            $data['FNAME'] = $user->first_name;
        }

if ('' !== $user->last_name) {
            $data['LNAME'] = $user->last_name;
        }

if ('' !== $user->first_name && '' !== $user->last_name) {
            $data['NAME'] = sprintf('%s %s', $user->first_name, $user->last_name);
        }

/**
         * @since 3.0
         * @deprecated 4.0 Use mc4wp_integration_user_data instead
         * @param array $data
         * @param WP_User $user
         */
        $data = (array) apply_filters('mc4wp_user_merge_vars', $data, $user);

/**
         * Filters the data for user-related integrations
         *
         * @since 4.2
         * @param array $data
         * @param WP_User $user
         */
        $data = (array) apply_filters('mc4wp_integration_user_data', $data, $user);

return $data;
    }
}

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