<?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"));
}