The phrase "for all someone knows/cares" is often used to express uncertainty or disinterest in a particular situation. There are various synonyms that can be used in place of this phrase. One such synonym is "as far as someone is concerned". This implies that the person in question has a limited knowledge or opinion on the matter. Another synonym is "in someone's eyes". This suggests that the perception of the situation is subjective and can vary depending on the individual. Similarly, the phrase "in someone's view" can also be used in this context. Overall, there are several alternatives to the phrase "for all someone knows/cares" that are equally effective in conveying a similar meaning.