<?php
$rp = new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::PRIMARY);
var_dump($rp->getModeString());
$rp = new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::PRIMARY_PREFERRED);
var_dump($rp->getModeString());
$rp = new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::SECONDARY);
var_dump($rp->getModeString());
$rp = new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::SECONDARY_PREFERRED);
var_dump($rp->getModeString());
$rp = new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::NEAREST);
var_dump($rp->getModeString());
?>
The above example will output:
string(7) "primary"
string(16) "primaryPreferred"
string(9) "secondary"
string(18) "secondaryPreferred"
string(7) "nearest"