Script problem

I have a script that I use on my site, LoveToText.com, and I don't know how to go about editing it.

Currently, if you don't enter a subject for example, the page will reload with a message saying 'No subject was provided.'. How do I change the code to make a seperate page load instead of the small error message?

The current code looks like this:

$ERROR1 = "No Message was provided.";
$ERROR2 = "No Cell Phone Number was provided.";
$ERROR3 = "No Email Address was provided.";
$ERROR4 = "No Subject was provided.";
$ERROR5 = "No Name was provided.";


Thanks in advance,

Ed.