Resource Types
Date/Time
Date and Time
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().

`SUNFUNCS_RET_TIMESTAMP` (integer)
  Timestamp
`SUNFUNCS_RET_STRING` (integer)
  Hours:minutes (example: 08:02)
`SUNFUNCS_RET_DOUBLE` (integer)
  Hours as floating point number (example 8.75)

Resource Types
Date/Time
Date and Time
PHP Manual