$foo = array( "bar1" => array(), 
  "bar2" => array() 
);

{ "bar1": {}, "bar2": []
}

{ "bar1": [], "bar2": []
}

{ "bar1": {}, "bar2": {}
}

$foo = array( "bar1" => new stdClass(), // Should be encoded as an object "bar2" => array() // Should be encoded as an array ); echo json_encode($foo);

$foo = array( "bar1" => (object)array(), // Should be encoded as an object "bar2" => array() // Should be encoded as an array ); echo json_encode($foo); // {"bar1":{}, "bar2":[]}