What do you want? A pocketmine version for minecraft 1.2 that works with php 7.0? Spoiler: Hint: That doesn't exist.
Why do you want to make 1.2 of PocketMine from PHP 7.2 to PHP 7.0.... You should know why they changed over to PHP 7.2 as well as you have to get PHP 7.2 nothing we can do or want to do about always updating for PHP 7.2 and 7.0. They PHP7.0 branch is probably the old PMMP (before switching to PHP 7.2). Isn't it better to use the latest version of stuff than crying and I even had to update tons of stuff on my Linux system from nginx to other processes that use PHP still it gives you experience and a good idea of Linux.
It's simple: You don't. Here's why: https://github.com/pmmp/PocketMine-MP/releases/tag/1.6.2dev-562 See that one line of text that says:
PHP 7.0 is not safe to use, there are thread-safety issues with it which are fixed in PHP 7.2. You're better off upgrading. If that's not enough, 7.2 brings lots of nice new language features and substantial performance improvements. TL;DR: we aren't supporting PHP 7.0 anymore.
Linux: https://jenkins.pmmp.io/job/PHP-7.2...cessfulBuild/artifact/PHP_Linux-x86_64.tar.gz Windows: https://ci.appveyor.com/api/buildjobs/ecg0htknihslg51t/artifacts/php-7.2.0RC2-vc15-x64.zip
Also make sure your PHP version is on 7.2 before adding the PHP 7.2 bin for PMMP. Linux Code: php -v I don't know about windows.
You don't need to change your system version, just run composer with your 7.2 PHP executable ("/bin/php7/bin/php")
Oh anyways I had updated my Linux Ubuntu 16.04 PHP System Version to 7.2 and then had used the PHP 7.2 Bin.