PHP code is embedded into the html source of a website, and is run on the web hosting server rather than on the browser. So, if you look at the source code of a PHP web page, the code that you see is just plain HTML. Since PHP is a universal scripting language, websites created with PHP are fully compatible with popular mobile devices like iPhone and iPad. A large number popular websites, including Facebook and Wikipedia, are written in PHP.
WordPress, Drupal, and Joomla are the most commonly-used open source content management systems written in PHP. A lot of webmasters choose these three platforms to build their websites because of their flexibility. There are a lot of themes and plugins created for these three platforms and webmasters can create professional looking websites based on these systems with just a few clicks. Also, their themes are customizable, so webmasters can change the coding to fit their needs.