A good question always have multiple answers, to reduce and help you choose the right answer, here I am adding my own too. I have tested it and it works fine.
AFHTTPRequestOperationManager *manager = [[AFHTTPRequestOperationManager alloc] initWithBaseURL:[NSURL URLWithString:@"http://www.yourdomain.com/appname/data/ws/index.php/user/login/"]];
manager.requestSerializer = [AFJSONRequestSerializer serializer];
manager.responseSerializer = [AFHTTPResponseSerializer serializer];
[manager POST:@"POST" parameters:parameters success:^(AFHTTPRequestOperation *operation, id responseObject) {
NSString *json = [[NSString alloc] initWithData:responseObject encoding:NSUTF8StringEncoding];
NSLog(@"%@", json);
//Now convert json string to dictionary.
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(@"%@", error.localizedDescription);
}];