There two main reasons for this change:

* Active support of PHP 5.x ended on January 2017, security support will
  end on December 2018. Packages in the main repository should be
  supported for at least 2 years, this means until first quarter of 2019
  for the upcoming v3.6.

* php7 and its extensions are currently in the community repository, so
  we can't use single abuild for both php5-* and php7-* packages (as we
  do for Python and Lua packages).

This change was suggested by @vakartel, approved by @ncopa, @kaniini, and
@jirutka.