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:

Note that if you are just adding things to the start/end of a page's body and don't need it to actually be integrated into the parsing process you should consider using a separate hook to add/prepend that content instead.