Using function like where clause

Hello,

I have 2 questions.
1) Can I use a function that will return a string in a where clause like
bellow?

select *
from table
where my_function_making_where()
and another_field = 'another_think'

2) Can I use a function that will return a string to return the list of
columns that I want to show like below?

select my_function_making_list_of_columns()
from table
where field_test = 'mydatum'

Thanks