PHP (a recursive acronym for PHP: Hypertext Preprocessor) is a computer scripting language, originally designed for producing dynamic web pages. It is for server-side scripting, but can be used from a command line interface or in standalone graphical applications.

While PHP was originally created by Rasmus Lerdorf in 1995, the main implementation of PHP is now produced by The PHP Group and serves as the de facto standard for PHP as there is no formal specification.[4] Released under the PHP License, the Free Software Foundation considers it to be free software

PHP is a widely-used general-purpose scripting language that is especially suited for web development and can be embedded into HTML. It generally runs on a web server, taking PHP code as its input and creating web pages as output. It can be deployed on most web servers and on almost every operating system and platform free of charge. PHP is installed on more than 20 million websites and 1 million web servers.[citation needed] It is also the most popular Apache module among computers using Apache as a web server. The most recent major release of PHP was version 5.2.6 on May 1, 2008.

PHP originally stood for Personal Home Page. It began in 1994 as a set of Common Gateway Interface binaries written in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf. Lerdorf initially created these Personal Home Page Tools to replace a small set of Perl scripts he had been using to maintain his personal homepage. The tools were used to perform tasks such as displaying his résumé and recording how much traffic his page was receiving.[4] He combined these binaries with his Form Interpreter to create PHP/FI, which had more functionality. PHP/FI included a larger C implementation and could communicate with databases enabling the building of simple, dynamic web applications. He released PHP publicly on June 8, 1995 to speed up the finding of bugs and improving the code.[9] This release was named PHP version 2 and already had the basic functionality that PHP has today. This included Perl-like variables, form handling, and the ability to embed HTML. The syntax was similar to Perl but was more limited, simpler, and less consistent

Zeev Suraski and Andi Gutmans, two Israeli developers at the Technion IIT, rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive initialism PHP: Hypertext Preprocessor. The development team officially released PHP/FI 2 in November 1997 after months of beta testing. Afterwards, public testing of PHP 3 began, and the official launch came in June 1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend Engine in 1999. They also founded Zend Technologies in Ramat Gan, Israel, which manages the development of PHP.

On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released.[4] On July 13, 2004, PHP 5 was released, powered by the new Zend Engine II.[4] PHP 5 included new features such as improved support for object-oriented programming, the PHP Data Objects extension (which defines a lightweight and consistent interface for accessing databases), and numerous performance enhancements.[11] The most recent update released by The PHP Group is for the older PHP version 4 code branch. As of January 2008, this branch is up to version 4.4.8. PHP 4 is no longer under active development but will be supported by security updates until August 8, 2008.

In 2008, PHP 5 became the only stable version under development. Late static binding has been missing from PHP and will be added in version 5.3.[14] [15] Alongside PHP 5, PHP 6 is also under active development. Major changes include the removal of register_globals[16], magic quotes, and safe mode.

PHP does not have complete native support for Unicode or multibyte strings;[18] unicode support will be included in PHP 6.[19] Many high profile open source projects ceased to support PHP 4 in new code as of February 5, 2008, due to the GoPHP5 initiative, provided by a consortium of PHP developers promoting the transition from PHP 4 to PHP 5.

It runs in both 32-bit and 64-bit environments, but on Windows the only official distribution is 32-bit, requiring Windows 32-bit compatibility mode to be enabled while using IIS in a 64-bit Windows environment. There is a third-party distribution[22] available for 64-bit Windows.

PHP is a general-purpose scripting language that is especially suited for web development. PHP generally runs on a web server, taking PHP code as its input and creating web pages as output. It can also be used for command-line scripting and client-side GUI applications. PHP can be deployed on most web servers, many operating systems and platforms, and can be used with many relational database management systems. It is available free of charge, and the PHP Group provides the complete source code for users to build, customize and extend for their own use.

PHP primarily acts as a filter[31], taking input from a file or stream containing text and/or PHP instructions and outputs another stream of data; most commonly the output will be HTML. It can automatically detect the language of the user.[32][33] From PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine, giving improved performance over its interpreter predecessor

Originally designed to create dynamic web pages, PHP's principal focus is server-side scripting[35], and it is similar to other server-side scripting languages that provide dynamic content from a web server to a client, such as Microsoft's ASP.NET system, Sun Microsystems' JavaServer Pages[36], and mod_perl. PHP has also attracted the development of many frameworks that provide building blocks and a design structure to promote rapid application development (RAD). Some of these include CakePHP, PRADO, Symfony and Zend Framework, offering features similar to other web application frameworks.

The LAMP architecture has become popular in the web industry as a way of deploying web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL, although the P may also refer to Python or Perl. As of April 2007, over 20 million Internet domains were hosted on servers with PHP installed, and PHP was recorded as the most popular Apache module

Insecure scripts written in PHP are popular targets of hackers who exploit poorly built applications written in PHP. Software vulnerabilities related to PHP are identified among the CVE (Common Vulnerabilities and Exposures) records, available from the National Vulnerability Database. The proportion of vulnerabilities related to PHP, out of the total of all common vulnerabilities, amounted to: 12% in 2003, 20% in 2004, 28% in 2005, 43% in 2006, 36% in 2007, and 33.8% for the first quarter of 2008. More than a quarter of all software vulnerabilities listed in this database are related to scripts written in PHP, and more than a third of vulnerabilities listed recently. Most of these vulnerabilities can be exploited remotely, that is without being logged on the computer hosting the vulnerable application.[37] Such exploitation is made possible due to poor programming habits, such as failing to check data before entering it into a database, and features of the language such as register_globals, which is now deprecated.[24] These result in code injection, cross-site scripting and other application security issues. Such attacks are not exclusive to PHP and most can be avoided simply by following proper coding techniques and principles.