bryansmith  

PHP/FI the original and first version was created by Rasmus Lerdorf way back in 1995. Over the span of 14 odd years and several evolutions it has become one of the most dominant programming languages for web development worldwide. Most of the open source web applications have embraced PHP as their language of choice.

Factors that make PHP so popular are several, some of them are:
• Ease of learning: the programming language constructs are easy to understand and there is plenty of material available online
• Abundance of freely available quality libraries: several quality reusable libraries of code have been made available on the net by php enthusiasts, allowing you to reuse code effectively
• LAMP (Linux, Apache, MySQL, PHP) hosting is cheap
• Availability of PHP based frameworks for rapid development (CakePHP)
• Ability to separate presentation layer from the business and data layer (Smarty Templates)
• Abundance of people carrying strong PHP development skillset
Some popular applications running on php are:
vBulletin, oscommerce, x-cart, pMachine, ez-Publish, gallery, phpBB, wordpress and php-nuke.

There have been several camps which have raised issues about PHP not being and enterprise grade software and not meant for high performance websites but all these claims have been baseless and biased views. Facebook.com is a good example of high traffic scalable site running on PHP.

Even though PHP can be used as a command line language, it's primary usage and acceptance is as a web development language.

If you are looking at one programming language to pick you might want to go with PHP as your choice.

For PHP Web Development needs to Hire PHP Programmers head there.