Muestra las etiquetas dentro de los campos del formulario de compra en la página «Finalizar compra» de WooCommerce.

add_filter( 'woocommerce_checkout_fields', 'wc_tags_inside', 9999 );
function wc_tags_inside( $fields ) {
    foreach ( $fields as $section => $section_fields ) {
        foreach ( $section_fields as $section_field => $section_field_settings ) {
            $fields[$section][$section_field]['placeholder'] = $fields[$section][$section_field]['label'];
            $fields[$section][$section_field]['label'] = '';
        }
    }
    return $fields;
}