PHP is a server-side scripting language because it runs on a web server and is used to generate dynamic web content. When a user visits a PHP web page in a browser, the web server interprets the PHP code and generates an HTML response, and then sends the response to the user's browser.
The underlying principle is that a protocol called "Common Gateway Interface" (CGI) is used between the web server and the PHP interpreter. When a user requests a PHP web page, the web server will start a PHP interpreter to process the request, and pass the requested data to the PHP interpreter in a specific format. The PHP interpreter will interpret the requested PHP code, generate the corresponding HTML response, and return the response to the web server, and finally the web server will send it to the user's browser.
Because PHP runs on the server side, PHP can access files, databases, and other resources on the server, generate dynamic content, and interact with other web services. This makes PHP a very popular language for web development, especially for developing dynamic websites, web applications, and e-commerce sites.