/** TC Kimlik No Ekleme **/

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

function custom_override_checkout_fields( $fields ) {
$fields['billing']['shipping_tc'] = array(
'label' => __('TC Kimlik No', 'woocommerce'),
'placeholder' => _x('Fatura İçin Gerekli', 'placeholder', 'woocommerce'),
'required' => true,
'class' => array('form-row-wide'),
'clear' => true
);

return $fields;
}

/** TC Doğrula  **/
function isTcKimlik($tc){
if(strlen($tc) < 11){ return false; }
if($tc[0] == '0'){ return false; }
$plus = ($tc[0] + $tc[2] + $tc[4] + $tc[6] + $tc[8]) * 7;
$minus = $plus - ($tc[1] + $tc[3] + $tc[5] + $tc[7]);
$mod = $minus % 10;
if($mod != $tc[9]){ return false; }
$all = '';
for($i = 0 ; $i < 10 ; $i++){ $all += $tc[$i]; }
if($all % 10 != $tc[10]){ return false; }

return true;
}

/**  TC Kimlik Noyu Doğrula **/
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
$tcno = $_POST['shipping_tc'];
if(!isTcKimlik($tcno))
wc_add_notice( __( 'Lütfen Geçerli TC Kimlik No Girin.' ), 'error' );
}

/** Admin Sipariş Detayında Fatura Bilgilerinde TC No'yu Görebilmesi İçin**/

add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );

function my_custom_checkout_field_display_admin_order_meta($order){
echo '<p><strong>'.__('TC Kimlik No').':</strong> ' . get_post_meta( $order->get_id(), '_shipping_tc', true ) . '</p>';
}