> How common is it in the wild? I am pretty sure I've used it myself without really thinking many times, which I suspect is common. Knowing how much code would be impacted would determine how we'd go about deprecating it and on what timeline.
I'm not sure how would be possible to find out. I suppose most of the frameworks (and thus popular PHP libraries) avoid this constant as showed in Drupal and Carbon. The scanning of popular libraries will not show the scale of the problem (however I will do it tomorrow) Helpful fact is that those constants are just constants so it's easily replaceable with strings 'Y-m-d\\TH:i:sO' and 'D, d M Y H:i:s \\G\\M\\T'.
