TYPO3 Core

You can use:

$_EXTKEY = $GLOBALS['_EXTKEY'] = 'YOUR_EXT_KEY';

seen in "EXT:news"(https://github.com/TYPO3-extensions/news/blob/master/Configuration/TCA/Overrides/tt_content.php)

Since the extension key is hardly changed, it's a good compromise.

OK, but what's the rationale behind it? Is $_EXTKEY and $GLOBALS going to be deprecatd/removed in ext_tables.php, too?

If a global variable is bad, why not provide something else, e.g. a function that returns the current extension key? Sure my extension knows how it's named, but I think it shouldn't necessarily need to. In any case, having $_EXTKEY greatly reduced search/replace effort for FluidTYPO3 provider extensions I need to copy, rename and modify for different sites.

I've thought about it again, and reached the conclusion that I can understand the decision and it wouldn't add much value after all.
Thanks for the quick reply.