Manual:Defines.php

Defines.php contains "a few constants that might be needed during LocalSettings.php. Note: these constants must all be resolvable at compile time by HipHop, since this file will not be executed during request startup for a compiled MediaWiki." Among the constants are version constants for the benefit of extensions; Unicode normalisation related constants; namespace constants, including those that provide support for $wgResourceModules; protocol constants for wfExpandUrl; and flags for Parser::replaceLinkHolders.