create or replace function f_test(pn_doc_id in integer) return integer is
   ln_res   integer;
   lv_stmt varchar2(255);
begin
  lv_stmt := '
    select count(*) into :ln_res from docs where doc_id = '||pn_doc_id||';
  ';
  EXECUTE IMMEDIATE lv_stmt USING IN OUT ln_res;
  
  return(ln_res);
end;

select f_test(5) from dual;

create or replace function f_test(pn_doc_id in integer) return integer is
   ln_res   integer;
   lv_stmt varchar2(255);
begin
  lv_stmt := 'select count(*) from docs where doc_id = :doc_id';
  EXECUTE IMMEDIATE lv_stmt into ln_res USING IN pn_doc_id;

  return ln_res;
end;