другой подход без циклов или поиска.
list($prev,$next) = getPrevNext($oObjects,$sCurrentKey);
function getPrevNext($aArray,$key){
$aKeys = array_keys($aArray); //every element of aKeys is obviously unique
$aIndices = array_flip($aKeys); //so array can be flipped without risk
$i = $aIndices[$key]; //index of key in aKeys
if ($i > 0) $prev = $aArray[$aKeys[$i-1]]; //use previous key in aArray
if ($i < count($aKeys)-1) $next = $aArray[$aKeys[$i+1]]; //use next key in aArray
return array($prev,$next);
}