I've created a new instance on Amazon's EC2 and I'm trying in wordpress and it's failing due to php-mbstring:

# yum install wordpress
Loaded plugins: amazon-id, rhui-lb
Resolving Dependencies
--> Running transaction check
---> Package wordpress.noarch 0:3.9.1-1.el7 will be installed
--> Processing Dependency: php-simplepie >= 1.3.1 for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-mbstring for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-gd for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-enchant for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-PHPMailer for package: wordpress-3.9.1-1.el7.noarch
--> Running transaction check
---> Package php-PHPMailer.noarch 0:5.2.6-1.el7 will be installed
--> Processing Dependency: php-mbstring >= 5.1.0 for package: php-PHPMailer-5.2.6-1.el7.noarch
---> Package php-gd.x86_64 0:5.4.16-21.el7 will be installed
--> Processing Dependency: libpng15.so.15(PNG15_0)(64bit) for package: php-gd-5.4.16-21.el7.x86_64
--> Processing Dependency: libt1.so.5()(64bit) for package: php-gd-5.4.16-21.el7.x86_64
--> Processing Dependency: libpng15.so.15()(64bit) for package: php-gd-5.4.16-21.el7.x86_64
--> Processing Dependency: libXpm.so.4()(64bit) for package: php-gd-5.4.16-21.el7.x86_64
--> Processing Dependency: libX11.so.6()(64bit) for package: php-gd-5.4.16-21.el7.x86_64
---> Package php-simplepie.noarch 0:1.3.1-4.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-simplepie-1.3.1-4.el7.noarch
--> Processing Dependency: php-IDNA_Convert for package: php-simplepie-1.3.1-4.el7.noarch
---> Package wordpress.noarch 0:3.9.1-1.el7 will be installed
--> Processing Dependency: php-mbstring for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-enchant for package: wordpress-3.9.1-1.el7.noarch
--> Running transaction check
---> Package libX11.x86_64 0:1.6.0-2.1.el7 will be installed
--> Processing Dependency: libX11-common = 1.6.0-2.1.el7 for package: libX11-1.6.0-2.1.el7.x86_64
--> Processing Dependency: libxcb.so.1()(64bit) for package: libX11-1.6.0-2.1.el7.x86_64
---> Package libXpm.x86_64 0:3.5.10-5.1.el7 will be installed
---> Package libpng.x86_64 2:1.5.13-5.el7 will be installed
---> Package php-IDNA_Convert.noarch 0:0.8.0-2.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-IDNA_Convert-0.8.0-2.el7.noarch
---> Package php-PHPMailer.noarch 0:5.2.6-1.el7 will be installed
--> Processing Dependency: php-mbstring >= 5.1.0 for package: php-PHPMailer-5.2.6-1.el7.noarch
---> Package php-simplepie.noarch 0:1.3.1-4.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-simplepie-1.3.1-4.el7.noarch
---> Package t1lib.x86_64 0:5.1.2-14.el7 will be installed
---> Package wordpress.noarch 0:3.9.1-1.el7 will be installed
--> Processing Dependency: php-mbstring for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-enchant for package: wordpress-3.9.1-1.el7.noarch
--> Running transaction check
---> Package libX11-common.noarch 0:1.6.0-2.1.el7 will be installed
---> Package libxcb.x86_64 0:1.9-5.el7 will be installed
--> Processing Dependency: libXau.so.6()(64bit) for package: libxcb-1.9-5.el7.x86_64
---> Package php-IDNA_Convert.noarch 0:0.8.0-2.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-IDNA_Convert-0.8.0-2.el7.noarch
---> Package php-PHPMailer.noarch 0:5.2.6-1.el7 will be installed
--> Processing Dependency: php-mbstring >= 5.1.0 for package: php-PHPMailer-5.2.6-1.el7.noarch
---> Package php-simplepie.noarch 0:1.3.1-4.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-simplepie-1.3.1-4.el7.noarch
---> Package wordpress.noarch 0:3.9.1-1.el7 will be installed
--> Processing Dependency: php-mbstring for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-enchant for package: wordpress-3.9.1-1.el7.noarch
--> Running transaction check
---> Package libXau.x86_64 0:1.0.8-2.1.el7 will be installed
---> Package php-IDNA_Convert.noarch 0:0.8.0-2.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-IDNA_Convert-0.8.0-2.el7.noarch
---> Package php-PHPMailer.noarch 0:5.2.6-1.el7 will be installed
--> Processing Dependency: php-mbstring >= 5.1.0 for package: php-PHPMailer-5.2.6-1.el7.noarch
---> Package php-simplepie.noarch 0:1.3.1-4.el7 will be installed
--> Processing Dependency: php-mbstring for package: php-simplepie-1.3.1-4.el7.noarch
---> Package wordpress.noarch 0:3.9.1-1.el7 will be installed
--> Processing Dependency: php-mbstring for package: wordpress-3.9.1-1.el7.noarch
--> Processing Dependency: php-enchant for package: wordpress-3.9.1-1.el7.noarch
--> Finished Dependency Resolution
Error: Package: php-PHPMailer-5.2.6-1.el7.noarch (epel)
           Requires: php-mbstring >= 5.1.0
Error: Package: php-IDNA_Convert-0.8.0-2.el7.noarch (epel)
           Requires: php-mbstring
Error: Package: wordpress-3.9.1-1.el7.noarch (epel)
           Requires: php-mbstring
Error: Package: php-simplepie-1.3.1-4.el7.noarch (epel)
           Requires: php-mbstring
Error: Package: wordpress-3.9.1-1.el7.noarch (epel)
           Requires: php-enchant
 You could try using --skip-broken to work around the problem
 You could try running: rpm -Va --nofiles --nodigest
# 

I'm using RHEL7:

# cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 7.0 (Maipo)
# yum repolist
Loaded plugins: amazon-id, rhui-lb
repo id                                                                                                     repo name                                                                                                                   status
epel/x86_64                                                                                                 Extra Packages for Enterprise Linux 7 - x86_64                                                                              4,325
rhui-REGION-client-config-server-7/x86_64                                                                   Red Hat Update Infrastructure 2.0 Client Configuration Server 7                                                                 1
rhui-REGION-rhel-server-releases/7Server/x86_64                                                             Red Hat Enterprise Linux Server 7 (RPMs)                                                                                    4,447
repolist: 8,773
# 

a while back and another environment I had to run following command first in order to get access to php-mbstring:

rhn-channel --add --channel=rhel-x86_64-server-optional-6

How do you do that in Amazon EC2?: