Add Billing fields on registration page

// Function to check starting char of a string
function startsWith($haystack, $needle){
    return $needle === '' || strpos($haystack, $needle) === 0;
}


// Custom function to display the Billing Address form to registration page
function zk_add_billing_form_to_registration(){
    global $woocommerce;
    $checkout = $woocommerce->checkout();
    ?>
    <?php foreach ( $checkout->get_checkout_fields( 'billing' ) as $key => $field ) : ?>

        <?php if($key!='billing_email'){ 
            woocommerce_form_field( $key, $field, $checkout->get_value( $key ) );
        } ?>

    <?php endforeach; 
}
add_action('woocommerce_register_form_start','zk_add_billing_form_to_registration');

// Custom function to save Usermeta or Billing Address of registered user
function zk_save_billing_address($user_id){
    global $woocommerce;
    $address = $_POST;
    foreach ($address as $key => $field){
        if(startsWith($key,'billing_')){
            // Condition to add firstname and last name to user meta table
            if($key == 'billing_first_name' || $key == 'billing_last_name'){
                $new_key = explode('billing_',$key);
                update_user_meta( $user_id, $new_key[1], $_POST[$key] );
            }
            update_user_meta( $user_id, $key, $_POST[$key] );
        }
    }

}
add_action('woocommerce_created_customer','zk_save_billing_address');


// Registration page billing address form Validation
function zk_validation_billing_address(){
    global $woocommerce;
    $address = $_POST;
    foreach ($address as $key => $field) :
        // Validation: Required fields
        if(startsWith($key,'billing_')){
            if($key == 'billing_country' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please select a country.', 'woocommerce' ) );
            }
            if($key == 'billing_first_name' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter first name.', 'woocommerce' ) );
            }
            if($key == 'billing_last_name' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter last name.', 'woocommerce' ) );
            }
            if($key == 'billing_address_1' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter address.', 'woocommerce' ) );
            }
            if($key == 'billing_city' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter city.', 'woocommerce' ) );
            }
            if($key == 'billing_state' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter state.', 'woocommerce' ) );
            }
            if($key == 'billing_postcode' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter a postcode.', 'woocommerce' ) );
            }
            /*
            if($key == 'billing_email' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter billing email address.', 'woocommerce' ) );
            }
            */
            if($key == 'billing_phone' && $field == ''){
                $woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter phone number.', 'woocommerce' ) );
            }

        }
    endforeach;
}
add_action('register_post','zk_validation_billing_address');