There is a mod which do this and SMF 2.0 succesfully work on php 7.0 with this mod.Actually no. This MOD only duplicates *-mysql.php to *-mysqli.php. Inside all functions are still *mysql* which causes fatal errors in php7.
So you basically need to properly duplicate mysql --> mysqli calls (and function calls have changed) and fix session handing changes.
It is being worked, but it requires more than meets the eye
I have a forum working on php 7.0 with this mod.
I second this request. If it isn't done in SMF 2.0.13 I know a lot of people who are going to find other forum software.
Let me just take the time to say this.... I feel that as a fellow developer myself (not an SMF dev, I don't know much PHP!), this really needs to be said. We work hard for you guys, we really do. You are the reason we exist. While we can't satisfy every need or request you make, we do our best. We do have our shortcomings and pitfalls, but in the end we are really here for you, and that is what matters most to us.
I have little to no knowledge of PHP 7, but I trust the SMF guys know enough of their codecase and PHP 7 to know what should work and shouldn't work. Just because you see a forum using a mod for PHP 7 doesn't mean it was easily done, nor does it show any headaches that may have come about while working on the mod. Apply that to the official version of SMF. You just simply can't come here and say it was done, without any knowledge of possible issues that may have been reached.
Anyways, why the big rush anyways? SMF works just fine the way it is, so I don't see why people would ditch the software if 2.0.13 isn't compatible with PHP 7... and don't hassle the developers either. People do that all the time, and there are misconceptions which are why that happens in the first place. Our job is fun, or it's boring, but it takes energy to do what we do.