PHP is a reflective programming language created for making dynamic Web pages. PHP is used mostly in server-side scripting, but it also can be used from a command line interface or in standalone graphical applications. PHP has gained popularity in recent times, and it is one of the forerunners in the Open Source software movement. Its popularity derives from its C-like syntax, its speed and its simplicity. PHP is currently divided into two major versions: PHP 4 and PHP 5, although PHP 4 is deprecated and is no longer developed or supplied with critical bug fixes. PHP 6 is currently under development.

PHP lets a static web document become dynamic. "PHP" is a recursive acronym standing for "PHP: Hypertext Preprocessor". PHP preprocesses hypertext documents. Because of this, the pages can change before the user sees them, based on conditions. This can be used to write something to the page, create a table with a number of rows equal to the number of times the user has visited, or integrate the web page with a web database, such as MySQL.

Before starting Server Side Processing, it is advised to have some basic understanding of the HyperText Markup Language. PHP is also being used to build GUI-driven applications; PHP-GTK is used to build Graphical User Interfaces.

PHP is a server-side scripting language that may be used on a host of web servers and platforms.  

The server-side scripting language means that the script is put into the HTML files that make up a site, but the server processes the script BEFORE it is sent to the client browser. PHP code is not visible if you view the source of a page because the server processes the code and returns only the output. This is easier to code and debug than writing CGI scripts in Perl or C since the HTML form and related code are all in one page and PHP puts any errors on the browser.

Another good thing that PHP offers is the possibility to directly connect to relational databases using f ull featured internal functions. It supports a whole fleet of databases comprising Oracle, DB2, mSQL and MySQL.

PHP's main focus is server-side scripting. While running the PHP parser with a web server and web browser, the PHP model can be compared to other server-side scripting languages such as Microsoft's ASP.NET system, Sun Microsystems' JavaServer Pages, mod_perl and the Ruby on Rails framework, as they all provide dynamic content to the client from a web server. To more directly compete with the "framework" approach taken by these systems, Zend is working on the Zend Framework - an emerging (as of June 2006) set of PHP building blocks and best practices; other PHP frameworks along the same lines include CakePHP, PRADO and Symfony.

The LAMP architecture has become famous in the Web industry as a way of deploying inexpensive, reliable, scalable, secure web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL. PHP can be used with a lot of relational database management systems, operates on all of the most popular web servers and is available for many different operating systems. This flexibility shows that PHP has a wide installation base across the Internet; over 19 million Internet domains are currently hosted on servers with PHP installed.

Examples of popular server-side PHP applications include phpBB, WordPress, Joomla and MediaWiki.