Sanitization callback for select and radio type Customizer controls.

customizer.php

<?php
/**
 * Sanitization callback for 'select' and 'radio' type controls.
 *
 * @copyright Copyright (c) 2015, WordPress Theme Review Team, https://github.com/WPTRT/code-examples
 * @see sanitize_key()               https://developer.wordpress.org/reference/functions/sanitize_key/
 * @see $wp_customize->get_control() https://developer.wordpress.org/reference/classes/wp_customize_manager/get_control/
 *
 * @param String               $input   Slug to sanitize.
 * @param WP_Customize_Setting $setting Setting instance.
 * @return string Sanitized slug if it is a valid choice; otherwise, the setting default.
 */
function theme_slug_sanitize_select( $input, $setting ) {

	// Ensure input is a slug.
	$input = sanitize_key( $input );

	// Get list of choices from the control associated with the setting.
	$choices = $setting->manager->get_control( $setting->id )->choices;

	// If the input is a valid key, return it; otherwise, return the default.
	return ( array_key_exists( $input, $choices ) ? $input : $setting->default );
}