Question

Why do we use PHP ?

Answer 1

Server-side scripting. This is the most traditional and main target field for PHP. You need three things to make this work. The PHP parser (CGI or server module), a web server and a web browser. You need to run the web server, with a connected PHP installation. You can access the PHP program output with a web browser, viewing the PHP page through the server. All these can run on your home machine if you are just experimenting with PHP programming. See the installation instructions section for more information.
Command line scripting. You can make a PHP script to run it without any server or browser. You only need the PHP parser to use it this way. This type of usage is ideal for scripts regularly executed using cron (on *nix or Linux) or Task Scheduler (on Windows). These scripts can also be used for simple text processing tasks. See the section about Command line usage of PHP for more information.
Writing desktop applications. PHP is probably not the very best language to create a desktop application with a graphical user interface,
Source : http://php.net/manual/en/intro-whatcando.php

Answer 2

PHP is an acronym for Personal Home Page.

Answer 3

PHP is used to manipulate, process and store data in a server while interacting with the client.

Answer 4

PHP is just one of the many server side web based languages out there. The use of a server side language in general is to give you the ability to "securely" manipulate and access data before sending it to the client. Imagine if everyone who accesed your website did so with the source code in JavaScript that lets them have direct access to your data; including the ability to manipulate it (delete and modify things) without constraint. If you want some advise learn C# and the .NET framework. It will make your life A LOT easier.