PHP is a general-purpose scripting language that is widely used for web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by the PHP Group. PHP was originally an abbreviation of Personal Home Page, but it now stands for the recursive initialism PHP: Hypertext Preprocessor.

PHP is executed on the server, generating HTML which is then sent to the client. PHP pages contain HTML with embedded code that does "something". The PHP code is enclosed in special start and end processing instructions <?php and ?> that allow you to jump into and out of "PHP mode". PHP files can contain text, HTML, CSS, JavaScript, and PHP code. PHP code can be used to generate dynamic page content, create, open, read, write, delete, and close files on the server, collect form data, send and receive cookies, add, delete, modify data in your database, control user-access, and encrypt data.

PHP is compatible with almost all servers used today. It is available on all major operating systems, such as Linux, Microsoft Windows, and macOS. Most web servers, including Apache and IIS, also support PHP. PHP can also be used for writing desktop graphical user interface (GUI) applications, by using the PHP-GTK extension.

In summary, PHP is a server-side scripting language that is widely used for web development. It is executed on the server and generates HTML which is then sent to the client. PHP files can contain text, HTML, CSS, JavaScript, and PHP code, and can be used to generate dynamic page content, control user-access, and encrypt data. PHP is compatible with almost all servers used today and is available on all major operating systems.