如何使用表单将表单外的变量发送到PHP文件？

I've got a page where I have a form that gets sent on to a php file that renders an email with the information and sends it on.

However, there are variables in the url (form.php?a=12&b=hello) that I also need to send.

I don't like the idea of having a hidden input field with this value or something like this. Is there a way to send it in a way that I can just $_REQUEST($a) and $_REQUEST($b) in the email rendering script?

Yes, you can. $_REQUEST contains both the $_POST and $_GET data, so you were already on the right track;

$_REQUEST['a']; // from the url
$_REQUEST['field_1']; // posted from <input name="field_1" />