The KashFlow PHP SDK by Joel Fielder is available to download, found in the tutorial "Integrating with Kashflow using PHP." References explain how to configure the KashFlow account, interact with the API, handle parameters, and create a sales invoice working with the API.
It's tax time and, if you're like me, you are going through your annual reflection, wondering if there's a better service you could use. There are a number of options available on the web for amateur bookkeepers. Here are a select few that have APIs.