> What do you mean? Do you mean that the constant was legal at some point?
Sorry, but I don't have this knowledge. It's invalid since 2004, but I suppose this format was implemented before then. Undoubtedly the format is invalid now. The usage in 1000 most popular libraries: DATE_ISO8601 (4 packages, 1 in comments) DateTimeInterface::ISO8601 (18 packages) DateTimeInterface::RFC7231 (1 package - Jetbrains/phpstorm-stubs) DATE_RFC7231 (1 package - Jetbrains/phpstorm-stubs) Most of the packages already inform about deprecation. PHP stubs by Jetbrans warn about deprecation, although it's not officially.
