The $IP variable is used in DefaultSettings.php, which means if you change it in LocalSettings.php, you must also override any DefaultSettings.php variable that depends on it.
It seems we we should either:
Version: 1.22.0
Severity: normal
$IP has already been used to load a bunch of code in WebStart.php long before LocalSettings.php is loaded -- it's WAY too late to change $IP there.
The only semi-correct way I can see to change it is to set the MW_INSTALL_PATH environment variable which can override the dirname(FILE) path, and I'm not 100% convinced that's actually useful for anything in the first place.
Comments in the code seem to indicate it can be used to override expansion of symlinks, but that seems like a wonky idea -- you've already hit at least index.php and includes/WebStart.php from the base directory, so you can't really flip to another version or something.
Given that we don't actually support changing $IP, should we move it to a constant instead like we recently did for MW_VERSION / $wgVersion?
Yes, though beware of mocking in tests. Code that isn't yet injecting its (in)direct use of $IP should likely continue to use the global.
Also, it'd be neat perhaps if we can align this with the ENV variable MW_INSTALL_PATH for consistency.