PHP Composer不会使用php 5.6安装Drush：phpunit / phpunit 4.8.x需要ext-dom

• php

so I'm working on CentOs 7 and I have been attempting to create a working Drupal Composer template (https://github.com/drupal-composer/drupal-project) that includes a site-local Drush install (http://docs.drush.org/en/master/install/). For our project we need to use PHP 5.6 and Drupal 8.2. 

Per the instructions above I have done the following on my clean virtualbox install of CentOS: 

Installed git.

Installed php 5.6:

1. rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

2. rpm -Uvh https://mirror.webtatic.com/yum/el7/webtatic-release.rpm

4. yum install php56w php56w-opcache

Now I download composer and install globally:

1. php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

2. php -r "if (hash_file('SHA384', 'composer-setup.php') === '669656bab3166a7aff8a7506b8cb2d1c292f042046c5a994c43155c0be6190fa0355160742ab2e1c88d40d5be660b410') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"

3. php composer-setup.php

5. mv composer.phar /usr/local/bin/compose

I can run composer and it seems to work fine.

Now per the instructions from Drupal Composer we can create our drupal project with composer using: 

composer create-project drupal-composer/drupal-project:8.x-dev some-dir --stability dev --no-interaction

The project template builds. 

However, my problem is that I cannot run 

composer require drush/drush

or

composer update

because I get these errors: 

1. Your requirements could not be resolved to an installable set of packages.

3. Problem 1

4. - phpunit/phpunit 4.8.x-dev requires ext-dom * -> the requested PHP extension dom is missing from your system.

5. - phpunit/phpunit 4.8.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.

6. - phpunit/phpunit 4.8.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.

7. - phpunit/phpunit 4.8.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.

8. - phpunit/phpunit 4.8.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.

9. - phpunit/phpunit 4.8.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.

10. - phpunit/phpunit 4.8.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.

11. - phpunit/phpunit 4.8.35 requires ext-dom * -> the requested PHP extension dom is missing from your system.

12. - phpunit/phpunit 4.8.34 requires ext-dom * -> the requested PHP extension dom is missing from your system.

13. - phpunit/phpunit 4.8.33 requires ext-dom * -> the requested PHP extension dom is missing from your system.

14. - phpunit/phpunit 4.8.32 requires ext-dom * -> the requested PHP extension dom is missing from your system.

15. - phpunit/phpunit 4.8.31 requires ext-dom * -> the requested PHP extension dom is missing from your system.

16. - phpunit/phpunit 4.8.30 requires ext-dom * -> the requested PHP extension dom is missing from your system.

17. - phpunit/phpunit 4.8.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.

18. - phpunit/phpunit 4.8.29 requires ext-dom * -> the requested PHP extension dom is missing from your system.

19. - phpunit/phpunit 4.8.28 requires ext-dom * -> the requested PHP extension dom is missing from your system.

20. - phpunit/phpunit 4.8.27 requires ext-dom * -> the requested PHP extension dom is missing from your system.

21. - phpunit/phpunit 4.8.26 requires ext-dom * -> the requested PHP extension dom is missing from your system.

22. - phpunit/phpunit 4.8.25 requires ext-dom * -> the requested PHP extension dom is missing from your system.

23. - phpunit/phpunit 4.8.24 requires ext-dom * -> the requested PHP extension dom is missing from your system.

24. - phpunit/phpunit 4.8.23 requires ext-dom * -> the requested PHP extension dom is missing from your system.

25. - phpunit/phpunit 4.8.22 requires ext-dom * -> the requested PHP extension dom is missing from your system.

26. - phpunit/phpunit 4.8.21 requires ext-dom * -> the requested PHP extension dom is missing from your system.

27. - phpunit/phpunit 4.8.20 requires ext-dom * -> the requested PHP extension dom is missing from your system.

28. - phpunit/phpunit 4.8.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.

29. - phpunit/phpunit 4.8.19 requires ext-dom * -> the requested PHP extension dom is missing from your system.

30. - phpunit/phpunit 4.8.18 requires ext-dom * -> the requested PHP extension dom is missing from your system.

31. - phpunit/phpunit 4.8.17 requires ext-dom * -> the requested PHP extension dom is missing from your system.

32. - phpunit/phpunit 4.8.16 requires ext-dom * -> the requested PHP extension dom is missing from your system.

33. - phpunit/phpunit 4.8.15 requires ext-dom * -> the requested PHP extension dom is missing from your system.

34. - phpunit/phpunit 4.8.14 requires ext-dom * -> the requested PHP extension dom is missing from your system.

35. - phpunit/phpunit 4.8.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.

36. - phpunit/phpunit 4.8.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.

37. - phpunit/phpunit 4.8.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.

38. - phpunit/phpunit 4.8.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.

39. - phpunit/phpunit 4.8.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.

40. - phpunit/phpunit 4.8.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.

41. - Installation request for phpunit/phpunit ~4.8 -> satisfiable by phpunit/phpunit[4.8.0, 4.8.1, 4.8.10, 4.8.11, 4.8.12, 4.8.13, 4.8.14, 4.8.15, 4.8.16, 4.8.17, 4.8.18, 4.8.19, 4.8.2, 4.8.20, 4.8.21, 4.8.22, 4.8.23, 4.8.24, 4.8.25, 4.8.26, 4.8.27, 4.8.28, 4.8.29, 4.8.3, 4.8.30, 4.8.31, 4.8.32, 4.8.33, 4.8.34, 4.8.35, 4.8.4, 4.8.5, 4.8.6, 4.8.7, 4.8.8, 4.8.9, 4.8.x-dev].

43. To enable extensions, verify that they are enabled in your .ini files:

44. - /etc/php.ini

45. - /etc/php.d/bz2.ini

46. - /etc/php.d/calendar.ini

47. - /etc/php.d/ctype.ini

48. - /etc/php.d/curl.ini

49. - /etc/php.d/exif.ini

50. - /etc/php.d/fileinfo.ini

51. - /etc/php.d/ftp.ini

52. - /etc/php.d/gettext.ini

53. - /etc/php.d/gmp.ini

54. - /etc/php.d/iconv.ini

55. - /etc/php.d/json.ini

56. - /etc/php.d/opcache.ini

57. - /etc/php.d/phar.ini

58. - /etc/php.d/shmop.ini

59. - /etc/php.d/simplexml.ini

60. - /etc/php.d/sockets.ini

61. - /etc/php.d/tokenizer.ini

62. - /etc/php.d/xml.ini

63. - /etc/php.d/zip.ini

64. You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Composer refuses to update. After a few days of googling, I seem to be at a loss. I don't know what exactly I need to change in those files, and I don't see anyone having the same error with their composer downloads (and most posts seem outdated by years). 

Can anyone offer any insight at all as to what I am missing here?

Thank you.

EDIT: Thanks to @Oliver I found out the issue was I needed php-xml which I installed for php5.6 using:

sudo yum install php56w-xml

Extensions can be found in the link in his comment.

Now I am figuring out what to do as I do not see drupal/core or easyrdf listed on that site. My new error is here: 

1. DrupalProject\composer\ScriptHandler::checkComposerVersion

2. Loading composer repositories with package information

3. Updating dependencies (including require-dev)

4. Your requirements could not be resolved to an installable set of packages.

6. Problem 1

7. - drupal/core 8.0.0 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

8. - drupal/core 8.0.0-beta10 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

9. - drupal/core 8.0.0-beta11 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

10. - drupal/core 8.0.0-beta12 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

11. - drupal/core 8.0.0-beta13 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

12. - drupal/core 8.0.0-beta14 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

13. - drupal/core 8.0.0-beta15 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

14. - drupal/core 8.0.0-beta16 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

15. - drupal/core 8.0.0-beta6 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

16. - drupal/core 8.0.0-beta7 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

17. - drupal/core 8.0.0-beta8 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

18. - drupal/core 8.0.0-beta9 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

19. - drupal/core 8.0.0-rc1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

20. - drupal/core 8.0.0-rc2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

21. - drupal/core 8.0.0-rc3 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

22. - drupal/core 8.0.0-rc4 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

23. - drupal/core 8.0.1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

24. - drupal/core 8.0.2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

25. - drupal/core 8.0.3 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

26. - drupal/core 8.0.4 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

27. - drupal/core 8.0.5 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

28. - drupal/core 8.0.6 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

29. - drupal/core 8.0.x-dev requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

30. - drupal/core 8.1.0 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

31. - drupal/core 8.1.0-beta1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

32. - drupal/core 8.1.0-beta2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

33. - drupal/core 8.1.0-rc1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

34. - drupal/core 8.1.1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

35. - drupal/core 8.1.10 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

36. - drupal/core 8.1.2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

37. - drupal/core 8.1.3 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

38. - drupal/core 8.1.4 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

39. - drupal/core 8.1.5 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

40. - drupal/core 8.1.6 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

41. - drupal/core 8.1.7 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

42. - drupal/core 8.1.8 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

43. - drupal/core 8.1.9 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

44. - drupal/core 8.1.x-dev requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

45. - drupal/core 8.2.0 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

46. - drupal/core 8.2.0-beta1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

47. - drupal/core 8.2.0-beta2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

48. - drupal/core 8.2.0-beta3 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

49. - drupal/core 8.2.0-rc1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

50. - drupal/core 8.2.0-rc2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

51. - drupal/core 8.2.1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

52. - drupal/core 8.2.2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

53. - drupal/core 8.2.3 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

54. - drupal/core 8.2.4 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

55. - drupal/core 8.2.5 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

56. - drupal/core 8.2.6 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

57. - drupal/core 8.2.7 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

58. - drupal/core 8.2.x-dev requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

59. - drupal/core 8.3.0-alpha1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

60. - drupal/core 8.3.0-beta1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

61. - drupal/core 8.3.0-rc1 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

62. - drupal/core 8.3.0-rc2 requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

63. - drupal/core 8.3.x-dev requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

64. - drupal/core 8.4.x-dev requires easyrdf/easyrdf 0.9.* -> satisfiable by easyrdf/easyrdf[0.9.0, 0.9.0-rc.1, 0.9.0-rc.2, 0.9.0-rc.3, 0.9.0-rc.4, 0.9.0-rc.5, 0.9.1, 0.9.x-dev].

65. - easyrdf/easyrdf 0.9.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

66. - easyrdf/easyrdf 0.9.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

67. - easyrdf/easyrdf 0.9.0-rc.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

68. - easyrdf/easyrdf 0.9.0-rc.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

69. - easyrdf/easyrdf 0.9.0-rc.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

70. - easyrdf/easyrdf 0.9.0-rc.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

71. - easyrdf/easyrdf 0.9.0-rc.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

72. - easyrdf/easyrdf 0.9.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

73. - Installation request for drupal/core ~8.0 -> satisfiable by drupal/core[8.0.0, 8.0.0-beta10, 8.0.0-beta11, 8.0.0-beta12, 8.0.0-beta13, 8.0.0-beta14, 8.0.0-beta15, 8.0.0-beta16, 8.0.0-beta6, 8.0.0-beta7, 8.0.0-beta8, 8.0.0-beta9, 8.0.0-rc1, 8.0.0-rc2, 8.0.0-rc3, 8.0.0-rc4, 8.0.1, 8.0.2, 8.0.3, 8.0.4, 8.0.5, 8.0.6, 8.0.x-dev, 8.1.0, 8.1.0-beta1, 8.1.0-beta2, 8.1.0-rc1, 8.1.1, 8.1.10, 8.1.2, 8.1.3, 8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.1.x-dev, 8.2.0, 8.2.0-beta1, 8.2.0-beta2, 8.2.0-beta3, 8.2.0-rc1, 8.2.0-rc2, 8.2.1, 8.2.2, 8.2.3, 8.2.4, 8.2.5, 8.2.6, 8.2.7, 8.2.x-dev, 8.3.0-alpha1, 8.3.0-beta1, 8.3.0-rc1, 8.3.0-rc2, 8.3.x-dev, 8.4.x-dev].

75. To enable extensions, verify that they are enabled in your .ini files:

76. - /etc/php.ini

77. - /etc/php.d/bz2.ini

78. - /etc/php.d/calendar.ini

79. - /etc/php.d/ctype.ini

80. - /etc/php.d/curl.ini

81. - /etc/php.d/dom.ini

82. - /etc/php.d/exif.ini

83. - /etc/php.d/fileinfo.ini

84. - /etc/php.d/ftp.ini

85. - /etc/php.d/gettext.ini

86. - /etc/php.d/gmp.ini

87. - /etc/php.d/iconv.ini

88. - /etc/php.d/json.ini

89. - /etc/php.d/opcache.ini

90. - /etc/php.d/phar.ini

91. - /etc/php.d/shmop.ini

92. - /etc/php.d/simplexml.ini

93. - /etc/php.d/sockets.ini

94. - /etc/php.d/tokenizer.ini

95. - /etc/php.d/xml.ini

96. - /etc/php.d/xml_wddx.ini

97. - /etc/php.d/xmlreader.ini

98. - /etc/php.d/xmlwriter.ini

99. - /etc/php.d/xsl.ini

100. - /etc/php.d/zip.ini

101. You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Will post update when if is figured out.

FINAL EDIT: Figured out that easyrdf needs ext-mbstring and ext-pcre. ext-pcre comes installed with the common php package, so all I needed was ext-mbstring:

sudo yum install php56w-mbstring

Now when calling composer update all my dependencies were downloaded! Now I can run vender/bin/drush and the command works, confirming my local install is completed!