DATE_RFC7231 constant missing

[roblourens]

There is a date constant, RFC7231 https://www.php.net/manual/en/class.datetimeinterface.php, which is missing from the list of date constants here: https://github.com/atom/language-php/blob/master/grammars/php.cson#L1385

Downstream: microsoft/vscode#157470

[KapitanOczywisty]

Multiple variables, functions and classes are missing, I'm working to extract all of them from php source.