WHMCS Accounting Software Electronic Invoice Integration

Back   Posted on 15 june 2019 / Updated on 24 january 2021
Reading time 5 minutes


Electronic Invoicing is spreading around the world and is quickly becoming mandatory in many countries. Billing Extension already integrates Italian E-Invoicing and soon also Slovenian one. Even though we would like to integrate all countries, this is prohibitive.

E-Invoicing differs enormously from country to country due to regulations and technological aspects. Let us put it into perspective. It took 4 months to complete the integration with Italy. We simply cannot repeat the same process tens of times. It would take years.

Instead of leaving non-Italian and non-Slovenian customers alone dealing with electronic invoicing, we came up with the idea of including a plugin in Billing Extension that provides all data you might need to integrate WHMCS E-Invoicing of any country.

We understand that the idea of having to complete the integration on your own doesn't sound good but at least you have something to start with. From our experience a big part of the job is preparing, validating and retrieving data from WHMCS and this plugin already gives you everything.

We underline that even if you don't need to comply to E-Invoicing regulations, this plugin can also be used for different purposes. For example you could use it to integrate WHMCS with your accounting software or intranet.

Getting Started

Begin by visiting Plugin section (Addons > Billing Extension > Settings and click "plus" icon) and activate WebService.


Once done go back to Settings and expand the newly added section. Here you find WebService configuration that at the moment only has one parameter named token. This randomly generated string is used to establish a secure connection for transmissions of data. Press the orange button to generate the token then hit save.

We prepared a PHP script with comments that explains how to interact with WebService (Download). It uses the same approach of WHMCS for External API requests via curl() library. Responses are in json format. A bit of PHP knowledge is required.


The autentication process is very simple and requires just two parameters.

Parameter Description Required
$URL URL to the root of WHMCS that can be found on Setup > General Settings > General > WHMCS SystemURL. Trailing slash "/" is required. Required
$Token It must be equal to the one you have in Addons > Billing Extension > Settings > WebService > Token Required


At the time of writing, this is the only supported action that can be used to retrieve billing details from WHMCS based on different criteria. We'll probably add more actions upon receiving feedback.

Request Parameters
Parameter Description Required
action “Get“ Required
start The starting date for the returned results. Supports YYYY-MM-DD dates, integers (5 returns last 5 days) and keywords (yesterday, month to date, last year etc.). Leave empty to get all invoices Optional
end If “start“ has been specified in date format, “end“ can be used to select invoices between a range of dates (eg. start 2019-06-01 end 2019-06-15). The only supported format is YYYY-MM-DD Optional
invoicenum Select the invoice with this specific Invoice Number. If in use, “start“ and “end“ values are ignored. Can't be used together with “invoiceid“ Optional
invoiceid Select the invoice with this specific Invoice ID. Must be an integer. If in use, “start“ and “end“ values are ignored. Can't be used together with “invoicenum“ Optional
doctype “Invoice“ and “CreditNote“ return invoices and credit notes respectively. If empty, both types are returned Optional
Response Parameters
Node Parameter Description
ClientData UserID User ID (tblclients.id)
ClientData ClientData Firstname (eg. “Jack“)
ClientData Lastname Lastname (eg. “Black“)
ClientData ClientName Firstname and lastname separated by space (eg. “Jack Black“)
ClientData CompanyName Company name
ClientData Email Email
ClientData Address1 Address 1
ClientData Address2 Address 2
ClientData City City
ClientData State State, region, province
ClientData PostCode Post code
ClientData Country Two-letter ISO country code (eg. IT, DE, ES)
ClientData PhoneNumber Phone number
ClientData Currency Currency ID of selected customer
ClientData TaxExempt “1“ is tax exempt. “0“ is not tax exempt
ClientData\CustomFields id Client custom field ID (tblcustomfields.id)
ClientData\CustomFields fieldname Client custom field name (eg. VAT Number)
ClientData\CustomFields value Client custom field value
ClientData\Europe MemberState Two-letter ISO country code (eg. IT, DE, ES)
ClientData\Europe Region Mostly “Europe“ but can also be equal to “South-America“, “Africa“ etc. for outermost regions and overseas territories of European Union
ClientData\Europe MonetaryUnion true/false (eg. Italy “true“, Denmark “false“)
ClientData\Europe VIES true/false. If “true“ the selected customer is an Intra-EU company registered on VIES
ClientData\Europe MOSS true/false
DocData Type “Invoice“ or “CreditNote“
DocData ID Document ID (tblinvoices.id)
DocData Num Document Number (eg. 2019-150)
DocData Status Invoice status (“Paid“, “Draft“, “Unpaid“ etc.)
DocData Date Date in YYYY-MM-DD format
DocData DueDate Due date in YYYY-MM-DD format
DocData DatePaid Date/time when invoice has been paid in YYYY-MM-DD hh:mm:ss format
DocData Subtotal Subtotal. 2 decimal places. Dot as the decimal separator
DocData Credit Credit. 2 decimal places. Dot as the decimal separator
DocData Tax Level 1 Tax. 2 decimal places. Dot as the decimal separator
DocData Tax2 Level 2 Tax. 2 decimal places. Dot as the decimal separator
DocData TaxRate Level 1 Tax Rate. 2 decimal places. Dot as the decimal separator
DocData TaxRate2 Level 2 Tax Rate. 2 decimal places. Dot as the decimal separator
DocData PaymentMethod Payment gateway (eg. paypal)
DocData\Items ID Invoice item ID (tblinvoiceitems.id)
DocData\Items Type “Setup“, “Hosting“, “Domain“, “Upgrade“, “Item“, “Addon“, “PromoHosting“, “DomainGraceFee“, “LateFee“ etc.
DocData\Items RelID ID of the related product/service, domain, addon, billing item etc.
DocData\Items Description Description (eg. Renew Domain example.com)
DocData\Items Amount Invoice line amount. 2 decimal places. Dot as the decimal separator
DocData\Items Taxed true/false

It is worth noting that ClientData values come from Invoice Snapshots therefore they're safe to use of billing purposes. Moreover such values are trimmed to remove whitespace from both ends of strings.


We made our WebService user-friendly. In practical terms when there's something wrong in your API request, WebService provides clear details about the error directly in the json response. Such errors are not cryptic but give a full description about what went wrong.

Date Keywords

When it comes to dates, the WebService supports special keywords that help you filtering records by predefined intervals listed in the table below. Monday is considered the first day of the week and the provided descriptions assume that current date is Saturday 2019-06-15.

Keyword Description
Today Select 2019-06-15
Yesterday Select 2019-06-14
Last 7 days Select from 2019-06-08 to 2019-06-15
Last week Select from 2019-06-10 to 2019-06-16
Week to date Select from 2019-06-10 to 2019-06-15
Month to date Select from 2019-06-01 to 2019-06-15
Previous month Select from 2019-05-01 to 2019-05-31
Year to date Select from 2019-01-01 to 2019-05-15
Last year Select from 2018-01-01 to 2018-12-31
All Select all records

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