validation callback

[eluser]quasiperfect[/eluser]
hi

i have two fields and i need to output a error based on the fields data.
something like the passwords that have to be equal but i don't need them to be eaqual but respect a custom relation.
how can i resolve this using a callback function ? i need to call callback_myfunc($param1,param2) to be able to compare the two values. or any other method to do this ?

thanks in advance for any response