PHP Manual

Predefined Constants

The following constants are defined since PHP 5.1.1 and they offer standard date representations, which can be used along with the date format functions (like date()).

DATE_ATOM (string)
Atom (example: 2005-08-15T15:52:01+00:00)
DATE_COOKIE (string)
HTTP Cookies (example: Monday, 15-Aug-05 15:52:01 UTC)
DATE_ISO8601 (string)
ISO-8601 (example: 2005-08-15T15:52:01+0000)
DATE_RFC822 (string)
RFC 822 (example: Mon, 15 Aug 05 15:52:01 +0000)
DATE_RFC850 (string)
RFC 850 (example: Monday, 15-Aug-05 15:52:01 UTC)
DATE_RFC1036 (string)
RFC 1036 (example: Mon, 15 Aug 05 15:52:01 +0000)
DATE_RFC1123 (string)
RFC 1123 (example: Mon, 15 Aug 2005 15:52:01 +0000)
DATE_RFC2822 (string)
RFC 2822 (Mon, 15 Aug 2005 15:52:01 +0000)
DATE_RFC3339 (string)
Same as DATE_ATOM (since PHP 5.1.3)
DATE_RSS (string)
RSS (Mon, 15 Aug 2005 15:52:01 +0000)
DATE_W3C (string)
World Wide Web Consortium (example: 2005-08-15T15:52:01+00:00)

Following constants exists since PHP 5.1.2 and specify a format returned by functions date_sunrise() and date_sunset().

Hours:minutes (example: 08:02)
Hours as floating point number (example 8.75)

PHP Manual