No Doubts.. PHP is the best language for web apps and softwares.

Some of the key advantages php has over others
1. Short learning curve. It is far easier for a beginner to learn php (tough to be expert might take some time).
2. 100% free
3. You could get help from unlimited communities from all over the world.
4. Around 60% of all Websites use some sort of PHP Code.
5. Most of the Billion dollar companies use PHP (Facebook,YouTube,Wikepedia and Flipkart ....)
6. Latest inclusion of OOPS concepts and php 6 give even more strength to php

Thank you for your answer, but can we build complete CMS with PHP or do we need any plugins...?

You can achive anything that you wish to be a web service you can be build with PHP.
Just go to github.com and search for some of the finest Open Source libraries for PHP.

I guess as far as "Plugins" you refer wordpress plugins or other CMSs .

Wordpress or Other CMS plugins can be made from scratch if you know some basics of php.

Always there are free versions available.
In software development world there is a golden rule
"Dont reinvent the WHEEL". Try to find the right plugin or a library already developed and tested. Customize it and improve it to your needs and proceed from there.
Thanks.

Thank a lot Dhayanandhan