Date/Time
PHP Manual

Predefined Constants

The DATE_ 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()).

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)

Date/Time
PHP Manual