the approach to manipulate the html code from your client is not the best, what I would consider is to make a php script that load the page and give in output the right portion....something server side....
However an approach is to use NSURLConnection or ASIHTTPRequest to download the page and then edit the stringResponse and then loading the result code with
Code:
[yourWebView loadHTMLString:htmlEdited baseURL:originalUrl];
but if you need to use cookies, get\post or any other advanced thing, can be hard to use this approach.