PHP

PHP (Recursive initialism for PHP: Hypertext Preprocessor) is a general purpose scripting language mainly used for back end programming for web servers, although it could be executed by the browser as well. It could process, receive and send HTTP requests. PHP has it's own interpreter and compiler.

A website theoretically could be 100% made by pure PHP syntax, although some static assets is needed for the PHP to fetch and get.

It is implemented on approximately 79.2% of all current web servers known popularly on the Internet.

History
PHP is written in the C programming language, although in it's early days, it is only created to serve blogs and news updates, it is now capable of multi purpose functions. The creator of PHP is Rasmus Lerdorf, and now maintained by the PHP team.

Syntax
Although PHP has a flexible and weak syntax, since version 7.0, it uses a gradual typing technique to type it's code.

The following is an example syntax of early PHP:
 * . The  syntax is used to fetch a file, in this case, in the root directory named.
 * . The  syntax is used to recognize the environment the user is browsing in, or the server's environment, in this case, to get the user browsing agent of the user.
 * . The  operator detects if the user agent is Mozilla, or not. If yes, continue to next line, if not, skip if section.
 * . End  statement.
 * . Select the current assigned variable from SQL database and check if the variable have a data cell in the username field.
 * . Skip  statement if numentries is 1.
 * . Exit preprocessor, go to content.
 * . Assign variables to $user and $index.

The explanation above is guessed due to the lack of PHP manuals for older syntaxes, please refer to the current PHP documentation and manual for updated view.