adding firstname and lastname on registration

Question

So i'm making a extension plugin FOR another plugin. What i want to achieve is that i upon registration they are required to add their Firstname and Lastname, so i added the fields for it, i did the errorchecks for it and everything worked when testing.

But when adding the last step the actual updating the database when i click the register button i get critical errors, the user is added to the database, but no first or lastname. I assume i don't even get in the "function registration_save_user".

Could someone help me on assisting me with my final hurdle, keep in mind i'm very new at all this so any improvements are welcomed. Thanks in advance.

add_filter( 'plugins_loaded', array( 'FootballPoolRegisterNames', 'init_extension' ) );

class FootballPoolRegisterNames {
    public static function init_extension() {
        add_action( 'register_form', [__CLASS__, 'registration_form_extra_names'], null, 2 );
        add_filter( 'registration_errors', [__CLASS__, 'registration_check_fields'], null, 3 );
        add_action( 'user_register', [__CLASS__, 'registration_save_user'], null, 4 );
    }
    
    //1. Add fields. Just the basic layout added before the dropdown.
    public static function registration_form_extra_names() {
        $first_name = ( ! empty( $_POST['first_name'] ) ) ? sanitize_text_field( $_POST['first_name'] ) : '';?>
        <p>
            <label for="first_name">Voornaam</label>
            <input type="text" name="first_name" id="first_name" class="input" value="<?php echo esc_attr(  $first_name  ); ?>" size="25" autocomplete="voornaam" required="required">
        </p>
        <?php
        $last_name = ( ! empty( $_POST['last_name'] ) ) ? sanitize_text_field( $_POST['last_name'] ) : '';?>
        <p>
            <label for="last_name">Achternaam</label>
            <input type="text" name="last_name" id="last_name" class="input" value="<?php echo esc_attr(  $last_name  ); ?>" size="25" autocomplete="achternaam" required="required">
        </p>
        <?php
    }

    //2. Check fields 
    private static function check_field( &$errors, $field, $message ) {
        if ( Football_Pool_Utils::post_string( $field ) === '' ) {
            $errors->add( "{$field}_error", "<strong>Fout:</strong> voer je {$message} in." );
        }
    }

    public static function registration_check_fields( $errors ) {
        // Check the added fields
        self::check_field( $errors, 'first_name', 'voornaam' );
        self::check_field( $errors, 'last_name', 'achternaam' );

        return $errors;
    }
    
    //3. Save userdata
    public static function registration_save_user( $user_id , $first_name, $last_name) {
        if ( ! empty( $_POST['first_name'] ) && ! empty( $_POST['last_name'] )) {
            update_user_meta( $user_id, 'first_name', $first_name );
            update_user_meta( $user_id, 'last_name', $last_name );
        }
    }
}

Answer 1

The user_register action hook only passes 2 parameters and your callback requires 3, which will result in a PHP fatal error. You should instead be using the insert_custom_user_meta filter like so:

class FootballPoolRegisterNames {
  public static function init_extension() {
    ...
    add_filter( 'insert_custom_user_meta', [ __CLASS__, 'insert_custom_user_meta' ], 10, 4 );
  }
  ...
  public static function insert_custom_user_meta( $custom_meta, $user, $update, $userdata ) {
    $custom_meta[ 'first_name' ] = sanitize_text_field( $_POST[ 'first_name' ] ?? '' );
    $custom_meta[ 'last_name' ] = sanitize_text_field( $_POST[ 'last_name' ] ?? '' );
    return $custom_meta;
  }
}