The mbstring extension for PHP is not a dependency to any of the already
listed packages on RHEL-like systems, and is needed by Phabricator
(showing a "install mbstring" message as the first thing if it is not
installed)
RHEL seems to have some extra steps to allow php-mbstring to be installed, though:
http://snippets.roozbehk.com/post/35750940300/php-mbstring-missing-on-red-hat-enterprise-linux-6
PS: disabled lint for this change because of the already >80 chars long "yum install" string