It seems like Dynamic\FoxyStripe\Model\FoxyCart::getStoreKey() can also be of type false; however, parameter $pwd of rc4crypt::decrypt() does only seem to accept string, 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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return new \SimpleXMLElement(\rc4crypt::decrypt(/** @scrutinizer ignore-type */ FoxyCart::getStoreKey(), $data));