We are a VAT registered company in UK, and I am the lead developer of a small team.
http://www.whatusersdo.com/
Like many online global businesses invoicing clients from many different countries, we are obliged to invoice them correctly with or without VAT whether they are VAT registered companies within EU or not, or individuals, etc.
So, we looked for a sensible solution, and implemented better and more accurate taxation for our invoicing module using the SOAP service from official EU website:
http://ec.europa.eu/taxation_customs/vies/vatRequest.html
Using a WSDL to PHP code generator, Wsdl2PhpGenerator, we could easily understand how it works and integrate it with our client sign up process behind the scenes, because we gather the country and VAT number of every client, if applicable.
We have used their public WSDL file here:
http://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl
I hope that highlights an important part of number crunching in many e-commerce apps and many developers may ignore 🙂