It seems like str_split(bin2hex(base64...pFieldType, $key))), 8) can also be of type true; however, parameter $pieces of implode() does only seem to accept array, 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 expression return $this->key could return the type null which is incompatible with the type-hinted return string. Consider adding an additional type-check to rule them out.