http://help.securepaynet.net/topic/421/article/510?prog_id=428178
Using the PHP Form-MailerTo use the PHP form-mailer, create a Web form in HTML that links to our PHP script, located in the root directory of your hosting account, and upload it to your hosting account.
Remember to specify the email address you want to use with the form-mailer in your account manager. For more information, see Specifying an Email Address for the PHP Form-Mailer.
To Use the PHP Form-Mailer
1. Create your Web form as normal and assign unique names to your form items.
NOTE: Keep in mind that our form-mailer script will sort the names of your form items alphabetically when it composes the email message. This is the order of precedence: uppercase letters, lowercase letters, numbers.
2. For the form action line, enter /gdform.php. For example:

3. Set the form method to “post.”
4. In addition to the fields you create in your form, there are three special fields that you can use: subject, redirect, and email.
Subject. Controls the subject line in the form email.
Redirect. Controls the page that your visitors will see after they submit the form.
Email. Controls the return address for the form email.
For example:

5. Save and upload your file to your hosting account.