It seems like $callback can also be of type null; however, parameter $callback of call_user_func() does only seem to accept callable, maybe add an additional type check?
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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if (call_user_func(/** @scrutinizer ignore-type */ $callback, $value, $key)) {