如何使用StringWithContentsForURL:编码:错误:?

- (id)initWithContentsOfURL:(NSURL *)url usedEncoding:(NSStringEncoding *)enc error:(NSError **)error

NSURL *url = [[NSURL alloc] initWithString:@"http://my_url.com/my_file.xml"];
NSError *error = nil;
NSStringEncoding encoding;
//NSString *my_string = [[NSString alloc] initWithContentsOfURL:url
//                                                     encoding:NSUTF8StringEncoding
//                                                        error:&error];
NSString *my_string = [[NSString alloc] initWithContentsOfURL:url
                                                 usedEncoding:&encoding 
                                                        error:&error];

NSURL *url = [[NSURL alloc] initWithString:@"http://my_url.com/my_file.xml"];

NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];
[request setURL:url];
[request setHTTPMethod:@"GET"];

NSURLConnection *conn = [[NSURLConnection alloc]initWithRequest:request delegate:self];

    [conn start];

if(conn){
    // Data Received
    responseData = [[NSMutableData alloc] init];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {

[self.responseData appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
     NSString *string = [[NSString alloc] initWithData:responseData encoding:NSASCIIStringEncoding];
}

header('Content-Type: text/html; charset=UTF-8');