sphinx multi valued filter

publn_date is multi-valued

<?php

ini_set('memory_limit', '-1');
ini_set('max_execution_time', '100');
include 'sphinxapi.php';

$sc= new SphinxClient();
$sc->setServer('localhost', 9312);
$sc->setRankingMode(SPH_RANK_NONE);
$sc->SetLimits(0,73180000,1000000);

$sc->SetFilterRange("publn_date", strtotime('1992-04-01'), strtotime('1998-02-01'), $exclude=false);

$sc->SetMatchMode(SPH_MATCH_EXTENDED2);
$res= $sc->query('@appln_abstract now');
$ids    = array_keys($res['matches']); 

print_r($ids);

?>