It doesn't matter if the server is PHP or any other technology. You can use NSURLRequest to submit it.
Anyway, if for some reason you need to use the UIWebView approach, you'll need to fill the form and submit it using JavaScript with stringByEvaluatingJavaScriptFromString.
Add something like this in -[UIWebViewDelegate webViewDidFinishLoad:]: (adjust jQuery paths to your case)
- (void)webViewDidFinishLoad:(UIWebView *)webView
{
...
NSString *result;
result = [webView stringByEvaluatingJavaScriptFromString:@"$('#username-box').val('the-user-name');"];
result = [webView stringByEvaluatingJavaScriptFromString:@"$('#password-box').val('secret');"];
result = [webView stringByEvaluatingJavaScriptFromString:@"$('#submit-button').click();"];
...
}