(PECL mongo >=1.3.0)
MongoDB::setReadPreference — Set the read preference for this database
$read_preference
[, array $tags
] )read_preferenceThe read preference mode: MongoClient::RP_PRIMARY, MongoClient::RP_PRIMARY_PREFERRED, MongoClient::RP_SECONDARY, MongoClient::RP_SECONDARY_PREFERRED, or MongoClient::RP_NEAREST.
tagsAn array of zero or more tag sets, where each tag set is itself an array of criteria used to match tags on replica set members.
Returns TRUE on success, or FALSE otherwise.
Example #1 MongoDB::setReadPreference() tag set array syntax example
<?php
$m = new MongoClient();
$db = $m->test;
// Prefer the nearest server in the "east" data center also used for reporting,
// but fall back to a server in the "west" data center
$db->setReadPreference(MongoClient::RP_NEAREST, array(
array('dc' => 'east', 'use' => 'reporting'),
array('dc' => 'west'),
));