>> This would require a yield to syntax, which doesn't exist.
There is however the 'yield from' statement.
This means, that most likely the keyword will not be "yield". PHP is not known for keeping a low number of keywords. Rather they introduce new ones, to tack on things to the language, like one can see in the implementation of anonymous functions, where one needs to use "use" explicitly, instead of the language having the semantics that most other languages have for lambdas. Which immediately turns in to a source for nullability errors.
That's not totally correct. PHP has a "short function" syntax for that specific use case, it automatically captures data without the 'use' statement.
$a = 5;
$b = fn ($c) => $a * $b;
print($b(2)); // 10
print($b(4)); // 20