Dynamic button creation using shortcodes

Hello,

I’ve been experimenting with the s2Member shortcode for generating buttons. I recognise that the shortcode returns HTML:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">



</form>

The encrypted value above is abbreviated.

If I include the shortcode in a PHP file, rather than on the membership signup page for example, how can I capture the value from the line:

In other words I want to capture value="—–BEGIN PKCS7—–MIIK…." and store it as a variable.
1) is this possible?
2) if yes – how?
3) is a PayPal Pro account with s2Member Pro a way around this?

I have been looking at similar topics (eg. http://www.s2member.com/kb/using-variables-in-a-shortcode/).

Thank you!