If this sounds familiar... it is. In the past phpBB team did similar things with PHP versions to force users to go to 3.1. Anyway, DionDesigns, wasn't willing to allow that to happen again. So he fixed the PHP 7.0 compatibility issues on phpBB 3.1.
The attached ZIP archive contains a program that will automatically patch your phpBB 3.1.9+ installation. It has some requirements to use, so please follow the instructions in the README.TXT file in the archive.
If you have problems which cannot be resolved, restore the original files, then restore the _bocodes table from the ACP Database Restore page. I would also appreciate your posting a note here describing your problems.
In addition to PHP 7.0 support, these files also add support for emojis if your version of PHP has the mbstring extension installed. This isn't the image-based junk in WordPress and phpBB 3.2, it's real emoji support. And it works without the need to mess with "mb4" collations in your database. That means you can add emojis in posts, forum descriptions, ranks, censored words... whatever. You can add them in forum names and topic titles as well, but support there is limited.