The global variable $IP should be replaced by a global constant MW_INSTALL_PATH. The advantages of the constant over the variable are not huge, but I think still compelling:
Change 757744 had a related patch set uploaded (by Daniel Kinzler; author: Daniel Kinzler):
[mediawiki/core@master] Define MW_INSTALL_PATH constant.
Change 759233 had a related patch set uploaded (by Daniel Kinzler; author: Daniel Kinzler):
[mediawiki/core@master] Remove usage of $IP from Language and friends.
Change 759238 had a related patch set uploaded (by Daniel Kinzler; author: Daniel Kinzler):
[mediawiki/core@master] Remove usage of $IP in some simple cases
Change 763217 had a related patch set uploaded (by Hashar; author: Hashar):
[integration/quibble@master] release: Quibble 1.4.1
Change 763217 merged by jenkins-bot:
[integration/quibble@master] release: Quibble 1.4.1
Mentioned in SAL (#wikimedia-releng) [2022-02-16T18:20:47Z] <hashar> Tag Quibble 1.4.1 @ d4bd2801de # T300301
Since Quibble 1.4.1 we define MW_INSTALL_PATH as a PHP constant and all Jenkins jobs have been switched :)
Change 757744 merged by jenkins-bot:
[mediawiki/core@master] Define MW_INSTALL_PATH constant and BaseDirectory config.
@daniel Making this a constant looks great. I don't understand the purpose of $wgBaseDirectory in addition to MW_INSTALL_PATH.
I think we'd need to set a pretty high bar of purpose and value to justify keepin it, as it is imho inevitable that this will result in confusion, plus its added value will likely be deminished by people not use them correctly.
I would like to replace its use with MW_INSTALL_PATH and remove it as a config variable.
The BaseDirectory config variable can be changed to a different values in phpunit tests. That's the only reason to have it, but I think it's a good reason.
Setup.php forces them to be the same initially. But I agree that it's a bit confusing.