We can use Predefined Constants for Date format in date() instead of the conventional date format strings since PHP 5.1.0.

Predefined Date Format Constants Available

DATE_ATOM - Atom (2016-07-22T14:50:01+00:00)

DATE_COOKIE - HTTP Cookies (Friday, 22-Jul-16 14:50:01 UTC)

DATE_RSS - RSS (Fri, 22 Jul 2016 14:50:01 +0000)

DATE_W3C - World Wide Web Consortium (2016-07-22T14:50:01+00:00)

DATE_ISO8601 - ISO-8601 (2016-07-22T14:50:01+0000)

DATE_RFC822 - RFC 822 (Fri, 22 Jul 16 14:50:01 +0000)

DATE_RFC850 - RFC 850 (Friday, 22-Jul-16 14:50:01 UTC)

DATE_RFC1036 - RFC 1036 (Fri, 22 Jul 16 14:50:01 +0000)

DATE_RFC1123 - RFC 1123 (Fri, 22 Jul 2016 14:50:01 +0000)

DATE_RFC2822 - RFC 2822 (Fri, 22 Jul 2016 14:50:01 +0000)

DATE_RFC3339 - Same as DATE_ATOM (2016-07-22T14:50:01+00:00)

Usage Examples

echo date(DATE_RFC822);

This will output: Fri, 22 Jul 16 14:50:01 +0000

echo date(DATE_ATOM,mktime(0,0,0,8,15,1947));

This will output: 1947-08-15T00:00:00+05:30