#PHP, 15 bytes, 2 exit codes

Without `die`/`exit`, PHP cannot return anything but `0` or `255` (afaik; probably `~1`), so ...

<!-- language: lang-php -->

	!$argv[1]?:p();

If command line argument is falsy, it evaluates to `1` and exits with `0`. If not, it tries to call a function and exits with `<b>Fatal error</b>: Uncaught Error: Call to undefined function p()`.

Run with `-r`.