like / as

                     ◊ Like is a preposition, used before a noun or pronoun, to say that two things are similar.
                       * He works like a slave.  
                       * They swim like fish.

                     ◊ As, as a preposition, before a noun or pronoun, is used to refer to jobs and functions.
                       * He works as an accountant.
                       * She was employed as a teacher.

                     ◊ As, as a conjunction, is used before subject + verb, or before a prepositional expression.
                       * As I said before, I will meet you at the airport.
                       * In the summer, as on Saturdays, the shop opens at 8 a.m.  

                       Compare :

                       I am your friend, and as your friend, I advise you to be careful.

                       I don't know you, but like your friend, I advise you to be careful.
                       ( = I agree with your friend, I think the same way).