How to revert back and undo changes in Centos?

For production use, the best approach is to have a testing environment where you can test any changes before applying them into production. Having this as a VM of course helps with snapshots and rollbacks.

Another approach is the use of modern file systems like ZFS that also allow to do snapshots or even LVM.

This is best done with in a virtual machine such as VMware or VirtualBox, rather than on physical hardware. Take a snapshot of your virtual machine prior to making any changes, then it's a trivial matter of reverting to your snapshot should you need to revert to your previous state.

Say you install some package:

yum install php

And say that package php installs a bunch of dependencies.

Dependencies Resolved

===============================================================================================================================
 Package                          Arch                       Version                            Repository                Size
===============================================================================================================================
Installing:
 php                              x86_64                     5.3.3-49.el6                       base                     1.1 M
Installing for dependencies:
 php-bcmath                       x86_64                     5.3.3-49.el6                       base                      40 k
 php-cli                          x86_64                     5.3.3-49.el6                       base                     2.2 M
 php-common                       x86_64                     5.3.3-49.el6                       base                     530 k
 php-dba                          x86_64                     5.3.3-49.el6                       base                      46 k
 php-devel                        x86_64                     5.3.3-49.el6                       base                     513 k
 php-embedded                     x86_64                     5.3.3-49.el6                       base                     1.1 M
 php-enchant                      x86_64                     5.3.3-49.el6                       base                      34 k
 php-gd                           x86_64                     5.3.3-49.el6                       base                     111 k
 php-imap                         x86_64                     5.3.3-49.el6                       base                      55 k
 php-intl                         x86_64                     5.3.3-49.el6                       base                      76 k
 php-ldap                         x86_64                     5.3.3-49.el6                       base                      43 k
 php-mbstring                     x86_64                     5.3.3-49.el6                       base                     460 k
 php-mysql                        x86_64                     5.3.3-49.el6                       base                      86 k
 php-odbc                         x86_64                     5.3.3-49.el6                       base                      56 k
 php-pdo                          x86_64                     5.3.3-49.el6                       base                      80 k
 php-pgsql                        x86_64                     5.3.3-49.el6                       base                      75 k
 php-process                      x86_64                     5.3.3-49.el6                       base                      44 k
 php-pspell                       x86_64                     5.3.3-49.el6                       base                      33 k
 php-recode                       x86_64                     5.3.3-49.el6                       base                      30 k
 php-snmp                         x86_64                     5.3.3-49.el6                       base                      36 k
 php-soap                         x86_64                     5.3.3-49.el6                       base                     145 k
 php-tidy                         x86_64                     5.3.3-49.el6                       base                      41 k
 php-xml                          x86_64                     5.3.3-49.el6                       base                     108 k
 php-xmlrpc                       x86_64                     5.3.3-49.el6                       base                      58 k
 php-zts                          x86_64                     5.3.3-49.el6                       base                     1.2 M

If you were to simply try to remove package php, via yum remove php, yum will do just that, and leave the most/all of the dependencies on the system since you didn't ask it to remove them as well.

Dependencies Resolved

===============================================================================================================================================================================================================
 Package                                                  Arch                                         Version                                               Repository                                   Size
===============================================================================================================================================================================================================
Removing:
 php                                                      x86_64                                       5.3.3-49.el6                                          @base                                       3.5 M
Removing for dependencies:
 php-devel                                                x86_64                                       5.3.3-49.el6                                          @base                                       3.0 M
 php-pecl-apc-devel                                       x86_64                                       3.1.9-2.el6                                           @base                                       3.3 k

Transaction Summary
===============================================================================================================================================================================================================
Remove        3 Package(s)

Rather, you can use yum to undo a transaction entirely, by invoking yum history command.

]# yum history
Loaded plugins: fastestmirror, security
ID     | Login user               | Date and time    | Action(s)      | Altered
-------------------------------------------------------------------------------
    41 |  <user>                 | 2018-06-08 10:45 | Install        |   26

And now undo history ID 41, in this case:

yum history undo 41

Note, that in some cases, yum won't be able to remove certain packages, if newer updates depend on them, or they are core system packages, etc... but generally, this will "undo" that transaction.

Also note, on Fedora (and soon on CentOS) systems, dnf package manager has the same commands, so dnf history will still work.

Last note, this won't undo any config file changes you've made in /etc (it will remove newly installed config files if a package put them there, however). Anything you do to the system will persist, anything the package manager did, will be reverted. If you need a full-system revert capability, then it's best to use a VM and snapshots as others have mentioned.