It seems like $url can also be of type string; however, parameter $boolean of abort_unless() does only seem to accept boolean, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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abort_unless(/** @scrutinizer ignore-type */ $url, 404, "Koel can't find such a song on iTunes Store.");
The expression return redirect($url) also could return the type Illuminate\Routing\Redirector which is incompatible with the documented return type Illuminate\Http\RedirectResponse.