Available from version 1.23.0 (Gerrit change 117840)
Is called before the MathML property is changed can be used e.g. for compression, normalization or introduction of custom hyperlinks etc.
Define function:
public static function onMathMLChanged( &$this , &$mathml ) { ... }
Attach hook: In extension.json:
	"Hooks": {
		"MathMLChanged": "MediaWiki\\Extension\\MyExtension\\Hooks::onMathMLChanged"
Called from: File(s):
Interface: MathMLChangedHook.php

For more information about attaching hooks, see Manual:Hooks .
For examples of extensions using this hook, see Category:MathMLChanged extensions.