PHP is a comprehensive programming language that allows increased functionality to be built into web sites.
PHP pages are usually called scripts, which is just another way of describing programmes.
PHP is a server side language, which means that the script is parsed by the PHP engine on the server before the page is downloaded to the browser.
A PHP script has a .php file extension as opposed the normal .html, or .htm, file extension.
This tells the web server software to pass the script to the PHP engine to parse the PHP code then strip out the PHP instructions before passing it back to the web server software for sending to the browser.
PHP instructions are usually embedded within HTML instructions and often write HTML elements and text content directly into the page.
This allows different versions of a page to be output depending on set conditions and is one of the features that makes PHP so useful.
PHP is also used to interface with online databases, such as MySQL, which allow a user's query to a database to be output as an HTML page. This feature allows very powerful web sites with increased functionality to be built.