Add the billing form to the registration page of woocommerce

billing_fields_to_registration.php

<?php

// Function to check starting char of a string
function zk_starts_with($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();
	foreach ( $checkout->get_checkout_fields( 'billing' ) as $key => $field ){ 
		if($key!='billing_email'){ 
			woocommerce_form_field( $key, $field, $checkout->get_value( $key ) );
		} 
	}
}
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 => $field){
		if(zk_starts_with($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 => $field){
		// Validation: Required fields
		if(zk_starts_with($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_phone' && $field == ''){
				$woocommerce->add_error( '<strong>' . __( 'ERROR', 'woocommerce' ) . '</strong>: ' . __( 'Please enter phone number.', 'woocommerce' ) );
			}
		
		}
	}
}
add_action('register_post','zk_validation_billing_address');