Description:
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There are a number of useful constants for formatting dates in PHP which cover most of the formats used on the internet. Unfortunately there is one missing format which can be pretty important, the one defined by RFC2616. 

All HTTP headers should follow this format. Because its a subset of RFC1123 that format is commonly substituted in PHP. This generally will work with permissive systems, however because the format provided by RFC1123 is system dependent it can provide strings that with more strict proxies will fail to parse. Generally that is UTC instead of the required GMT timezone. And old php.net documentation comment mentions this as well http://php.net/gmdate#25031.

Because of this Drupal has a custom constant to be used in headers https://www.drupal.org/node/1918820

However because of PHP's audience I think it would be a handy feature for everyone to have the additional format constant.

RFC2616 is actually obsoleeted by RFC7231 in 2014 so I would use this name as Drupal did but the format is the same.

https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3
http://tools.ietf.org/html/rfc7231#section-7.1.1.1