Quick test to confirm that two instances of MongoClient to separate hosts but using the same replicaSet will direct all queries to the cluster which was first instantiated.

MongoClient_Test.php

<?php
 
echo "<pre>";
echo " ######### Both instances connect to the same set of servers: \n\n";
 
$mc1 = new MongoClient('mongodb://rs1.example.com,rs2.example.com,rs3.example.com', [ 'replicaSet' => 'my_set' ] );
$mc2 = new MongoClient('mongodb://rep1.example.com,rep2.example.com,rep3.example.com', [ 'replicaSet' => 'my_set' ] );
 
echo " ######### MongoClient - ReplicaSet 1: \n";
print_r( $mc1->listDBs() ); // Shows the correct Dbs
 
echo " ######### MongoClient - ReplicaSet 2: \n";
print_r( $mc2->listDBs() ); // Incorrectly shows the Dbs from $mc1
 
 
echo " ######### Both still show the same Hosts and Connections to all hosts: \n\n";
 
$mc1 = new MongoClient('mongodb://rs1.example.com,rs2.example.com,rs3.example.com', [ 'replicaSet' => 'my_set' ] );
$mc2 = new MongoClient('mongodb://rep1.example.com');
 
echo " ######### MongoClient - ReplicaSet 1: \n";
print_r( $mc1->getHosts() );
print_r( $mc1->getConnections() );
print_r( $mc1->listDBs() ); // Shows the correct Dbs
 
echo " ######### MongoClient - Primary Only: \n";
print_r( $mc2->getHosts() );
print_r( $mc2->getConnections() );
print_r( $mc2->listDBs() ); // Now shows correct Dbs