(PECL mongo >=1.1.0)
MongoCollection::setSlaveOkay — Change slaveOkay setting for this collection
$ok = true
  ] )See the query section of this manual for information on distributing reads to slaves.
ok
     If reads should be sent to secondary members of a replica set for all possible queries using this MongoCollection instance.
Returns the former value of slaveOkay for this instance.