PHP is a widely used open source scripting language that is executed on the server. PHP files can contain text, HTML, CSS, JavaScript and PHP code. The PHP code is executed on the server and returned to the browser as HTML. PHP files have a .php extension and can run on various platforms. PHP supports databases and is easy to learn, running efficiently on the server side. PHP was designed to work with HTML and can be embedded into HTML code or used in pure PHP files without HTML tags. Basic PHP syntax follows rules like being case sensitive and requiring semicolons to end statements.