First and Last Name on the Registration Form

add_action( 'register_form', function () {
    $first = isset( $_POST['first_name'] ) ? sanitize_text_field( wp_unslash( $_POST['first_name'] ) ) : '';
    $last  = isset( $_POST['last_name'] ) ? sanitize_text_field( wp_unslash( $_POST['last_name'] ) ) : '';
    ?>
    <p>
        <label for="wpide_first_name">First name</label>
        <input type="text" name="first_name" id="wpide_first_name" class="input" value="<?php echo esc_attr( $first ); ?>" autocomplete="given-name" required>
    </p>
    <p>
        <label for="wpide_last_name">Last name</label>
        <input type="text" name="last_name" id="wpide_last_name" class="input" value="<?php echo esc_attr( $last ); ?>" autocomplete="family-name" required>
    </p>
    <?php
} );

add_filter( 'registration_errors', function ( $errors ) {
    if ( empty( $_POST['first_name'] ) || empty( $_POST['last_name'] ) ) {
        $errors->add( 'wpide_names', '<strong>Error:</strong> Please enter your first and last name.' );
    }

    return $errors;
} );

add_action( 'user_register', function ( $user_id ) {
    if ( ! empty( $_POST['first_name'] ) && ! empty( $_POST['last_name'] ) && 'wp-login.php' === ( isset( $GLOBALS['pagenow'] ) ? $GLOBALS['pagenow'] : '' ) ) {
        update_user_meta( $user_id, 'first_name', sanitize_text_field( wp_unslash( $_POST['first_name'] ) ) );
        update_user_meta( $user_id, 'last_name', sanitize_text_field( wp_unslash( $_POST['last_name'] ) ) );
    }
} );