<?php include_once 'include/init.php'; //include particular file for entity you need (Client, Invoice, Category...) include_once LIB_PATH . "/FreshBooks/Invoice.php"; $invoiceId = 4; //new Invoice $invoice = new FreshBooks_Invoice(); //try to get invoice id: $invoiceId if (!$invoice->get($invoiceId)) { //no data - read error echo $invoice->lastError; } else { //investigate populated data print_r($invoice); print_r($invoice->sendByEmail('Test invoice', "Hello")); }