使所有PHP-mongoDB连接都使用Unix域套接字

Since we started using mongoDB a while back, a large part of our PHP codebase looks like this:

$mongo = new Mongo();
$coll = $mongo->mydb->mycoll;

The default is to connect via TCP to localhost:27017 which has worked just fine for us for a few years now. Due to speed considerations, I would like to switch to using Unix domain sockets which are supported like this:

$client = new MongoClient("mongodb:///tmp/mongodb-27017.sock");

Is there a way to make this work without having to specify the socket file in the code? The docs list mongo.default_host and mongo.default_port which can be set in php.ini so you can write:

$client = new MongoClient();

but that seems to be possible only for TCP connections. Or can I use these parameters to specify a socket file? If so, how?