It seems like $this->readonlyProperties can also be of type null; however, parameter $search of array_key_exists() does only seem to accept array, maybe add an additional type check?
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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if (array_key_exists($name, /** @scrutinizer ignore-type */ $this->readonlyProperties)) {
The property readonlyProperties does not exist on GloBee\PaymentApi\Models\PropertyTrait. Since you implemented __get, consider adding a @property annotation.