Drush install on Redhat 7 Errors PHP Installing drush is never the same on every linux system apparently. I'm on redhat 7 and and I just setup php 5.6 and installed composer, but I can't do a composer install on the fresh git repo of drush. 

I'm getting the following error:

Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.

      Problem 1
        - Installation request for d11wtq/boris v1.0.10 -&gt; satisfiable by d11wtq/boris[v1.0.10].
        - d11wtq/boris v1.0.10 requires ext-posix * -&gt; the requested PHP extension posix is missing from your system.
      Problem 2
        - Installation request for phpunit/phpunit 4.6.6 -&gt; satisfiable by phpunit/phpunit[4.6.6].
        - phpunit/phpunit 4.6.6 requires ext-dom * -&gt; the requested PHP extension dom is missing from your system.


It's complaining about phpunit not being installed. So I went ahead and installed it:


  PHPUnit 5.6.2 by Sebastian Bergmann and contributors.


And I still get the same error when doing a composer install.

Can someone please explain why this always happens? You try to install something and it complains something isn't installed. You install that thing, but it still complains. Or better yet, you install something and it complains that something isn't installed and you install that thing and it complains some other thing isn't installed. Wasn't these package managers suppose to solve these dependency issues?
The errors are pretty self-explanatory:

The requested PHP extension posix is missing from your system
The requested PHP extension dom is missing from your system

You need to install those extensions because packages that drush depends on in turn depend on them.