Since GIT#99cff51099 one doesn't not have to specify any fixup for string or any type of variable type arguments. Everything will be converted to string by the core (but note that vars will not be auto-converted if quoted, if you need that you probably need one of the fixup_var_str_12() or fixup_var_pve_str_12()). E.g.: f($a, "$a = " + $a) will work automatically. For ints, either convert the string argument to int by hand, or use fixup_var_int_12() as fixup and then access the args with get_int_fparam().