Array-based filter

wp-signup.php

/**
 * Allow definition of default variables
 *
 * @since MU
 *
 * @param array $user_data The user data to define {
 *     @type string "user_name"  The username
 *     @type string "user_email" The user's email
 *     @type array  "errors"
 * }
 */
$filtered_results = apply_filters('signup_user_init', array('user_name' => $user_name, 'user_email' => $user_email, 'errors' => $errors ));
$user_name = $filtered_results['user_name'];
$user_email = $filtered_results['user_email'];
$errors = $filtered_results['errors'];