Here's the modification to get all the leads that were updated between noon and 3:45pm on 08/02/2013. Please modify as required
// Create Request
$leadSel = new stdClass();
// $leadSel->keyType = 'EMAIL';
// $keyValues = array("formtest1@marketo.com", "joe@marketo.com");
// $leadKeys = new stdClass();
// $leadKeys->latestUpdatedAt = $keyValues;
// $leadSel->keyValues = $leadKeys;
$leadSel->latestUpdatedAt = "2013-08-02T15:45:00-07:00";
$leadSel->oldestUpdatedAt = "2013-08-02T12:00:00-07:00";
$leadSelSoap = new stdClass();
$leadSelSoap = array("leadSelector" => $leadSel);
// $leadSelParams = array("leadSelector" => $leadSelSoap, "batchSize" => 10, "streamPosition" => $startPosition);
// $params = array("paramsGetMultipleLeads" => $leadSelParams);
$leadSelSoap = new SoapVar($leadSel, SOAP_ENC_OBJECT, "LastUpdateAtSelector", "http://www.marketo.com/mktows/");
$params = new stdClass();
$params->leadSelector = $leadSelSoap;
$params->batchSize = 100;
$soapClient = new SoapClient($marketoSoapEndPoint ."?WSDL", $options);
try {
$leads = $soapClient->__soapCall('getMultipleLeads', array($params), $options, $authHdr);
}