multiple-select-setting-callback.php

/* Multiple select */
function prefix_settings_multiple_select( $args ) {
	$selected_terms = get_option( $args['label_for'] );
	$selected_terms = is_array( $selected_terms ) ? $selected_terms : array();
	$selected_terms = array_map( 'absint', $selected_terms );

	printf(
		'<select name="%s" id="%s" multiple>',
		esc_attr( $args['label_for'] . '[]' ),
		esc_attr( $args['label_for'] )
	);

	$query = new WP_Query( array(
		'post_type'      => 'some_post_type',
		'posts_per_page' => 50,
	) );

	if ( $query->have_posts() ) {
		while ( $query->have_posts() ) {
			$query->the_post();

			printf(
				'<option value="%s" %s>%s</option>',
				esc_attr( get_the_ID() ),
				selected( in_array( get_the_ID(), $selected_terms, true ), true, false ),
				esc_html( get_the_title() )
			);
		}

		wp_reset_postdata();
	}

	printf( '</select>' );
}