PHP is the web development language written by and for web developers.
PHP stands for PHP: Hypertext Preprocessor. The product was originally named Personal Home Page Tools, and many people still think that’s what the acronym stands for, but as it expanded in scope, a new and more appropriate (albeit GNU-ishly recursive) name was selected by community vote.
PHP is currently in its sixth major rewrite, called PHP6 or just plain PHP. PHP is a server-side scripting language, usually used to create web applications in combination with a web server, such as Apache. PHP can also be used to create command-line scripts akin to Perl or shell scripts, but such use is much less common than PHP’s use as a web language.
Strictly speaking, PHP has nothing to do with layout, events, on-the-fly Document Object Model (DOM) manipulation, or really anything about the look and feel of a web page. In fact, most of what PHP does is invisible to the end user. Someone looking at a PHP page will not necessarily be able to tell that it was not written purely in Hypertext Markup Language (HTML), because the result of PHP is usually HTML.