PHP is a scripting language that was designed primarily for the purpose to design and create dynamic websites. It is also possible to use PHP as a command line tool or as as an independent component of a graphical application.
Forefather of PHP was Rasmus Lerdorf who created the first version in 1995. Current implementations are now being produced by the PHP Group.
Since there is no formal specification of the language itself the version represents a De-Facto-Standard. The PHP software is distributed under the specially created "PHP License" and is basically free. The PHP license is incompatible with the better-known GNU license. The main difference concerns the use of the term "PHP".
PHP is widely spread and finds its main usage in the development of websites.
PHP-language code can be inserted directly into an HTML document but it can only interpreted if a PHP-enabled webserver is present.
The Web server receives a request from the client's browser for a HTML document, interperts the PHP commands and sends the generated HTML page to the client.
The document with the PHP commands is not sent here. However, if the PHP code contains output statements, they appear at the exact place in the HTML where the PHP code was inserted. So for security reasons the user of the client PC will never see the PHP code.
With this technique the HTML code itself is changed.
Nowadays, many sites generate the whole content of their individual pages dynamically. The browser requests a page from the Web server, which is a pure PHP script that gets its content from databases and associated user settings in cookies, connecting to different HTML templates, and at the end a dynamically generated HTML document is sent back to the user.
This dynamism in the production of websites not only provides a simplified modularised development for the Creator, that is usually associated with significant time savings, but also an opportunity to hide parts of a site from the browser. The user of a Web site receives a single document that was produced with a wide variety of puzzle pieces. The production itself, for example what information has been used here, can not be followed precisely for technical and security reasons. Thus, PHP is used indirectly to protect intellectual property.
If you want to change the code of a PHP document requesting the file via HTTP will not help you but with a file transfer protocol called FTP it is possible to store PHP files locally and upload the changed version to the server.
There are several FTP programms available that offer a wide range of data manipulation.
The PHP code is interoperable in any case. A PHP script that was developed under Linux will also run on a Web server on Windows if the version of PHP are the same.
The latest version is 5.3.2 and with version 5.4 a lot of improvements and changes will be made regarding the object oriented structure.
Today, there are over 20 million PHP websites, connected to more than 1 million web servers.