Edit:

A new separate RFC for a void return type has been published, has passed the vote, and was implemented in PHP 7.1.
There is now a void return type in PHP. 🙂

Original Post:

Taken from wiki.php.net:

Future Work

Ideas for future work which are out of the scope of this RFC include:

• Allow functions to declare that they do not return anything at all (void in Java and C)

So currently there is no way to declare that you don’t return anything.
I don’t know what’s best in your situation, but I’d probably just go with not declaring the return type for now.

To answer your question whether there will be a void return type in PHP 7:
There is no guarantee yet, but I think it is very likely that void or a synonym will be implemented in some way.