customize-select-multiple.php

<?php
// @todo The following does not work.

add_action( 'customize_register', function( WP_Customize_Manager $wp_customize ) {
	$choices = array(
		'foo' => __( 'Foo' ),
		'bar' => __( 'Bar' ),
		'baz' => __( 'Baz' ),
	);

	$wp_customize->add_setting(  'favorite_meta_vars', array(
		'sanitize_callback' => function( $values ) use ( $choices ) {
			if ( ! is_array( $choices ) ) {
				return new WP_Error( 'invalid_value_type', __( 'Invalid value type.', '...' ) );
			}
			$allowed_options = array_keys( $choices );
			$values = array_values( $values );
			foreach ( $values as $value ) {
				if ( in_array( $value, $allowed_options, true ) ) {
					return new WP_Error( 'illegal_value', __( 'Illegal value.', '...' ) );
				}
			}
			return $values;
		},
	) );
	$wp_customize->add_control( 'favorite_meta_vars', array(
		'label' => __( 'Favorite meta var(s)', '...' ),
		'type' => 'select',
		'choices' => $choices,
		'setting' => 'favorite_meta_vars',
		'section' => 'title_tagline',
		'input_attrs' => array( // @todo The select control doesn't read from input_attrs.
			'multiple' => true,
		),
		'multiple' => true, // @todo Support this.
	) );
} );