./setup.sh

/**
 * Register settings
 *
 * @param array $settings Registered settings.
 */
function register_sections( $settings ) {
  $sections['reference_key'] = array(
    'option_group' => 'option_group_name',
    'option_name'  => 'db_key',
  );

  return $settings;
}

add_filter( 'FUNCTION_PREFIX_settings', 'register_sections', 10, 1 );

/**
 * Register sections
 *
 * @param array $sections Registered sections.
 */
function register_sections( $sections ) {
  $sections['new_section'] = array(
    'title' => __( 'Section Name', 'PLUGIN_TEXT_DOMAIN' ),
    'page'  => 'page_slug',
  );

  return $sections;
}

add_filter( 'FUNCTION_PREFIX_setting_sections', 'register_sections', 10, 1 );

function register_setting_fields( $settings_fields ) {
  $settings_fields['setting_key'] = array(
    'title'       => __( 'Setting Title', 'PLUGIN_TEXT_DOMAIN' ),
    'desc'        => sprintf(
      wp_kses(
        __( 'Setting description', 'PLUGIN_TEXT_DOMAIN' ),
        array()
      )
    ),
    'page'        => 'page_slude',
    'section'     => 'new_section',
    'option_name' => 'db_key',
    'type'        => 'text', // text, url, number, textarea.
    'field_class' => 'large-text', // large-text, medium-text, small-text, regular-text, code.
    'placeholder' => __( 'Placeholder text', 'PLUGIN_TEXT_DOMAIN' ),
    'value'       => ! empty( $options['setting_key'] ) ? $options['setting_key'] : false,
  );

  return $settings_fields;
}

add_filter( 'FUNCTION_PREFIX_settings_fields', 'register_setting_fields', 10, 1 );