PHP (Hypertext Preprocessor) is a free open source general-purpose scripting language that is widely used for developing web applications. It was originally designed to power websites written using the HTML 4.01 standard. PHP supports object-oriented programming features, including classes, interfaces, exceptions, and reflection. It includes many built-in data types, string handling, regular expressions, file manipulation, encryption/decryption, date/time conversion, XML parsing, and DOM manipulation.