Heroku already supported PHP via buildpacks,i've been using heroku and php for years,it works very well(a bit expensive though),but I ended up reproducing git deployement on my own servers.
We've hired a full time PHP owner. We've got PHP docs on the devcenter https://devcenter.heroku.com/articles/php-support and more importantly the features we document are "supported". If you've got problems with them we will provide application specific support (to the extent of supporting our docs and the platform). It also means that the experience will continue to get better as new features are added, and that PHP has a voice in the future of our platform.
PHP on Heroku has been a highly requested feature, and we're investing a good amount to make the experience a good one.
Even if i require a package through Composer, there is no guarantee i'll have the proper native extension installed(think mongodb ,intl,apc...),w hereas with ruby or node,their respective package managers actually install all packages,including C/C++ ones.
How will you manage that in PHP?