PayPal\Api\Invoice::generateNumber PHP Method

    public static function generateNumber($apiContext = null, $restCall = null)
    {
        $payLoad = "";
        $json = self::executeCall("/v1/invoicing/invoices/next-invoice-number", "POST", $payLoad, null, $apiContext, $restCall);
        $ret = new InvoiceNumber();
        $ret->fromJson($json);
        return $ret;
    }

<?php

// # Get Next Invoice Number Sample
// This sample code demonstrate how you can retrieve
// the next invoice number.
require '../bootstrap.php';
use PayPal\Api\Invoice;
// ### Get Next Invoice Number
// To generate the successive invoice number for the merchant, use below code.
// (See bootstrap.php for more on `ApiContext`)
try {
    $number = Invoice::generateNumber($apiContext);
} catch (Exception $ex) {
    // NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
    ResultPrinter::printError("Get Next Invoice Number", "InvoiceNumber", null, null, $ex);
    exit(1);
}
// NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
ResultPrinter::printResult("Get Next Invoice Number", "InvoiceNumber", null, $number, $number);
return $number;