Heroku has dropped support for PHP 5.5 in their official buildpack.
This is a fork of v127 of Heroku's PHP Buildpack, which has support for PHP 5.5
This has only been tested on Dokku, and is not guaranteed to work on Heroku.
Add Heroku PHP Buildpack (v127) as a dependency in your application:
composer require heroku/heroku-buildpack-php:127
Set the BUILDPACK_URL:
BUILDPACK_URL=https://github.com/devkokov/heroku-buildpack-php55