He got burnt.
He got burned.
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Both are forms of the Past Participle of the verb "burn". See for instance here.
I think that nowadays there is a tendency toward using regular verb forms, so "burned" is more frequent. Although, there's something to be said about the difference between "burnt" and "burned", similar to those forms of the verb "hang". A toast can be "burnt" when it's served to you for breakfast, but the passive voice is likely to involve the form "burned".