PHP (Hypertext Pre-Processor) is a general purpose server-side scripting language. PHP was designed for web development for producing dynamic web pages. We embed PHP code in static html pages in order to make them dynamic pages. Using PHP, connection between a database can be established, you manipulate on files, design classes. PHP has also evolved to include command line interface capability and it can be used in standalone graphical applications.
To be simple, we use PHP to make changes in pages. Consider for example, you fill a form for registration.
Now, after completing filling of your registration form you click submit to get registered to the website. Now, your processing of filling the page is a static page work. After submitting the registration form dynamic side of the action takes place in the server side. Client just makes a request to the server, server processes the request and responds to the request. So, dynamic part of the webserver i.e.., PHP, comes into action and does processes such as updating your details into the database. So, the actual processes that is going to take place after you enter the details is that, your details are entered into database of the server. Using PHP we can perform many of the automation processes.