Manual:$wgRequestTime

Overview
This is not an object variable, but is a global initialized by MediaWiki.

is a global microtime float that is initialized early in the engine that a lot of other times are computed relative to for logging, profiling, and debugging.

Examples
A snippet from [https://doc.wikimedia.org/mediawiki-core/master/php/html/Debug_8php_source.html Debug.php]: