After presenting the form to the user to fill in the amount (or other vars), send the data through the following code:
$plaintext_querystring="amount=".$amount."¤cy=".$currency."&payee_bcaddr=".$payee_bc_addr."&payee_name=".$payee_name."¬e=".$note."&success_url=".$success_url."&cancel_url=".$cancel_url."&baggage=".$baggage;
$result=mbc_encryptformdata($plaintext_querystring);
$redirect_url="https://www.mybitcoin.com/sci/paypage.php?t=".$result['SCI Encrypted Form Data Token'];
header("Location: $redirect_url");
You can also get most vars from your own database/cart and fill in the blanks from there.