How to detect user agent of iphone or ipad?

How to detect user agent of iphone or ipad?

I am using the following code to parse a remote XML :
NSString *urldata=[[NSString
alloc]initWithFormat:@"http://url.com/test/a.php"];
NSString *encode=[urldata
stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSLog(@" urldata %@",urldata);
TBXML *tbxml =[TBXML tbxmlWithURL:[NSURL URLWithString:encode]];
And my php script has $useragent= $_SERVER['HTTP_USER_AGENT'] ; . I am
unable to determine the user agent. However if call the url from safari
ipad or iphone I have the user agent correctly.