<?php
$m = new \MongoDB\Client(
    'mongodb://localhost:13000/?replicaSet=poiset' .
    '&readPreference=nearest&readPreferenceTags=dc:asia'
);

$m->demo->test->find( $query );
?>
or per-query:

<?php
$m = new \MongoDB\Client(
    'mongodb://localhost:13000/?replicaSet=poiset'
);

$rp = new MongoDB\Driver\ReadPreference(
    MongoDB\Driver\ReadPreference::RP_NEAREST, [ [ 'dc' => 'asia' ] ]
);

$m->demo->test->find( $query, [ 'readPreference' => $rp ] );
?>