World Wide Web Consortium (W3C)
The W3C, or World Wide Web Consortium, is the international standards organization for the World Wide Web. It publishes and maintains specifications for HTML, CSS, the DOM, accessibility guidelines such as WCAG, and many other web technologies.
W3C standards are not PHP-specific, but they shape how PHP applications produce and consume web content: the HTML a PHP script generates is expected to be valid against W3C’s HTML specification, and features like the PHP DOM extension implement the same DOM model that the W3C defines for browsers.