modify_xml

$xml = <<<EOD
        <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ar="http://ar.gov.afip.dif.FEV1/"> 
                    <soapenv:Header/> 
                    <soapenv:Body> 
                        <ar:FECompUltimoAutorizado> 
                            <ar:Auth> 
                                <ar:Token>string</ar:Token> 
                                <ar:Sign>string</ar:Sign> 
                                <ar:Cuit>long</ar:Cuit> 
                            </ar:Auth> 
                                <ar:PtoVta>int</ar:PtoVta> 
                                <ar:CbteTipo>int</ar:CbteTipo> 
                        </ar:FECompUltimoAutorizado> 
                    </soapenv:Body> 
                </soapenv:Envelope>
        EOD;

        $dom = new DOMDocument();
        $dom->loadXML($xml);

        $ptoVta = $dom->getElementsByTagName('ar:PtoVta')[0];

        $ptoVta->textContent = '10';

        $xml = $dom->saveXML();

        echo $xml;