Manual:Hooks/ParserBeforeStrip

Details
Note that this hook is not run on cached page content.

And is also run when parsing system messages in addition to the page text.

$text will hold the text being parsed.

To change the text being parsed, modify this value.

E.g. to add the phrase "The mighty oracle gives forth this proclamation: " to the front of the text being parsed, you would use the following code: