We love whitespace.
MediaWiki favors a heavily-spaced style for optimum readability.
Therefore, it seems like a no-brainer to use function () : type.
Specifically, https://www.mediawiki.org/wiki/Manual_talk:Coding_conventions/PHP/Archive#Declaring_of_return_types was archived by @Krinkle because he decided to collapse all the talk pages onto one.
In the absence of consensus, how about we just go with spaces for now?