Manual:$wgVersion

Details
The current version of MediaWiki (e.g, ""). You shouldn't need to set this option; it is used internally.

MW_VERSION
Starting with MediaWiki 1.35,   is deprecated, and replaced with the   constant. It is also available in MediaWiki 1.31.7, 1.33.3 and 1.34.1, though this should not be relied on as its presence is required for some release tools.   will continue to function the same as before in those versions.

Example code
If you're implementing a new feature that only works in MediaWiki 1.19 or later, but you want to maintain backwards compatibility, you could use a test such as the following:

However, it is typically recommended that you use feature detection to see whether the method or class you wish to use is available instead of depending upon raw version strings.