PHP began in 1994 when Rasmus Lerdorf wrote a series of common gateway interface Perl scripts to help run his personal homepage by displaying his resume, tracking web traffic, etc. From there it has expanded into major industry use as a server-side scripting language for making webpages more dynamic without involving the client-side. The newest release of PHP was in 2013 and it is currently installed on more than 244 million websites and 2.1 million web servers1. The name 'PHP' is something of a joke - it is a recursive acronym that standing for PHP: Hypertext Processor.
PHP operates by being interpreted by a web server with its specific processor module installed and then determining how to generate a webpage by reading the PHP code. PHP code can be standalone, but it is commonly embedded in HTML between delimiters. For example, should you come across something like
<?php echo 'Hello World'; ?> in the midst of some HTML, you would know you have stumbled upon a PHP block. In the case of embedding like this, the interpreter is blind to content outside the PHP delimiter tags (
<?php ?> are common, but a couple variations exist), though that content may change depending on what the recognizable PHP blocks dictate.
Despite its original purpose, PHP has evolved so that it does not need to be solely used within a web server. It can act as a general purpose language and now includes command-line interface capabilities, as well as some aptitude for standalone graphical applications.
1. Ide, Andy (2013-01-31). "PHP just grows & grows". Retrieved 2013-04-01 from http://news.netcraft.com/archives/2013/01/31/php-just-grows-grows.html.