The question is fairly straightforward: is the passive voice of "I know him," which would be "he is known by me," a valid English sentence? I thought I had heard/read it before, but I may be wrong and it may also be some form of slang.
I am a student of Italian, and in learning the passive voice, I attempted to make this same connection and was told that this is not a valid passive voice in Italian because "to know" ("sapere" in Italian for those interested) cannot be used in the passive voice.
This idea is mentioned in this question, but I struggled to find a definitive answer.
Best Answer
As Tᴚoɯɐuo points out in his comment, while "He is known by me" is grammatically correct, it's not something a native speaker would normally say. Instead the more idiomatic expression is "He is known to me".
This sentence is not necessarily the passive voice, but at some point you should recognize that every language has odd phrases and expressions that don't always match what you would expect from textbook grammar.
A variation of the passive "to be known" is sometimes used when talking about things rather than people:
Here there is no need to define who this is known to, since "widely" implies "by most people".