The hook documentation pages on mediawiki.org each contain an infobox which gives an example function definition for the hook.
I've noticed that a lot of the function signatures specify arguments that should be passed by reference, for example: https://www.mediawiki.org/wiki/Manual:Hooks/ArticleDeleteComplete
It feels that in a lot of cases this by-reference designation is unnecessary, as the argument is an object; objects are always passed by reference in PHP5. I suspect this is a hold-over from when MediaWiki supported PHP4, where the & was necessary to ensure a reference was passed, rather than a copy of the object.
The documentation should be updated to contain the correct function signatures for modern PHP. This will also help make it clearer which properties are actually intended for modification by the callback.
This will require some familiarity with the hooks in question, so probably requires developer input.