Create a PHP interface towards the following api:
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With your PHP interface we should be able to create a new invoice from a template. Some of the input parameters to your method should be: Amount, customer details, etc. We should be able to utilize your interface to send all sorts of variables. After the invoice has been created it should be downloaded (please check whether this is possible). Alternatively you can send the invoice via email to the customer. This last solution requires, that we (after paying Zoho for an upgrade) will be able to send it from our email address and completely customize the email content.