If I understand it right what you're trying to do, I'd say that it is not possible before the form gets submitted. The form values are on the client side whereas PHP is running on the server-side. Any processing would need to be done by the script specified in the
ACTION attribute. But then it's no problem - just put the mail command in that script.
You could plug in your script as the action and then you have two choices
1. Make a confirmation screen. Email all you need to yourself and then have another submit button at the bottom to resub the info to the other script. Have a titla similar to "You ebtered this info please click submit to send it".