Hello it looks like a great plugin however it does say that you need to have PHP 5.3 or higher installed in your sever as per the message below.
The Two Factor Auth plugin requires PHP version 5.3 or higher.
Will you release a version that caters for those who have not yet upgraded to the latest PHP version? Or is this the default settings, in this case perhaps if you don't mind add a note on the FAQ or at the description page.
Thank you kindly