It's been done before, so certainly do-able. I think you need to use something called nusoap (I'm not a PHP expert i'm afraid)
Ah... I had to do some digging and look at specific examples of the API but the system uses SOAP transactions to handle everything in which case you could use SOAP by configuring it in your PHP build. Ref: http://uk3.php.net/manual/en/soap.installation.php If you're distributing an app that can be installed locally to an end users machine though then the nusoap library looks to be quite useful..
nusoap should only be used for PHP 4.x. Seeing that most hosts now offer PHP 5.x, I would use the native PHP Soap functions within PHP 5
For those who are searching for php code The example below requires nusoap.php file PHP: <?phprequire_once('nusoap.php');$client = new soapclient('https://secure.kashflow.co.uk/api/service.asmx?WSDL','wsdl');$client->setDefaultRpcParams(true);$soapproxy = $client->getProxy();$username = "yourusername";$password = "yourpassword";$arr = array('UserName'=>$username,'Password'=>$password,'CustomerCode'=>'somecustomercode');$result=$soapproxy->GetCustomer($arr);print_r($result); //the result returned is in form of an array?>