add_action('rest_api_init', function () {
    register_rest_route('wpsync/v1', '/add-user', [
        'methods'  => 'POST',
        'callback' => function($request) {
            global $wpdb;
            $table = $wpdb->prefix . 'custom_users';

            // Decode incoming JSON
            $data = json_decode($request->get_body(), true);

            // Minimal required fields
            $email = sanitize_email($data['email'] ?? '');
            $first = sanitize_text_field($data['first_name'] ?? '');
            $last  = sanitize_text_field($data['last_name'] ?? '');
            $phone = sanitize_text_field($data['phone'] ?? '');

            if (!$email || !$phone) {
                return ['error' => 'Missing email or phone'];
            }

            // Create WP user
            $username = sanitize_user(strtolower($first . '.' . $last));
            $user_id = wp_create_user($username, wp_hash_password($phone), $email);

            if (is_wp_error($user_id)) {
                return ['error' => $user_id->get_error_message()];
            }

            // Insert into custom table
            $wpdb->insert(
                $table,
                [
                    'wp_user_id'    => $user_id,
                    'username'      => $username,
                    'email'         => $email,
                    'password_hash' => wp_hash_password($phone),
                    'first_name'    => $first,
                    'last_name'     => $last,
                    'display_name'  => trim("$first $last"),
                    'created_at'    => current_time('mysql'),
                ],
                ['%d','%s','%s','%s','%s','%s','%s','%s']
            );

            if ($wpdb->last_error) {
                return ['db_error' => $wpdb->last_error];
            }

            return ['status' => 'created'];
        },
        'permission_callback' => '__return_true',
    ]);
});
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/triaug5/public_html/wp-content/plugins/WPSYNC_test/WPSYNC_test.php:1) in <b>/home/triaug5/public_html/wp-includes/pluggable.php</b> on line <b>1531</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/triaug5/public_html/wp-content/plugins/WPSYNC_test/WPSYNC_test.php:1) in <b>/home/triaug5/public_html/wp-includes/pluggable.php</b> on line <b>1534</b><br />