I had the same question and thanks to the great work of @Kolohe with the mcrypt example I found a solution with openssl_decrypt and DES encryption.
With the upgrade to php7 we replaced mcrypt with openssl. So mcrypt wasn't a solution for us.
Because we pass the encrypted string as GET parameter it is wrapped with base64.
That's the AMPScript:
set @email = 'firstname.lastname@example.org'
set @email_code = Base64Encode(EncryptSymmetric(@email, "des;mode=ecb;padding=zeros", @null, 'ab12cd34'))
That's the result from the AMPScript:
That's the PHP part:
$crypted = 'TklMUTk4UjBhVlRWeXdDWHJCYk92Zz09';
$crypted = base64_decode($crypted);
$decrypted = openssl_decrypt($crypted, 'des-ecb', 'ab12cd34', OPENSSL_ZERO_PADDING, '');
$decrypted = trim($decrypted);
And the PHP result:
The decrypted string has 2 control charactes at the end. I don't know why and in the end I wrapped it with a trim to solve the problem. It seems to work.