A pull request is already made for this on feb 22nd: https://github.com/constantcontact/php-sdk/pull/36
So far no one has merged this. Without the modified_date property you can't sync contacts since you can't tell which one (local or remote) was modified last.
I want to offer my apologies for the delay in our response! This pull request was merged on Tuesday the 27th and the changes are available on GItHub.