updating and searching mysql set using php

updating and searching mysql set using php

I have a column in table which has possible values of tom,john,jerry(SET).
I have a variable that a user enter and I store it in $var and want that
at some point. I could check that if the value in $var exists in
particular row then don't update else update, How I can do this in PHP. I
have several rows like tom,john
john,jerry
I dont want `tom,john,tom`
Any help in this regard