It seems like $xml can also be of type false; however, parameter $xml of EndelWar\GestPayWS\Respo...Response::__construct() does only seem to accept SimpleXMLElement, 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
The property CryptDecryptString does not exist on EndelWar\GestPayWS\Response\EncryptResponse. Since you implemented __get, consider adding a @property annotation.