calling exec on a php file and passing parameters?

I am wanting to call a php file using exec.

When I call it I want to be able to pass a variable through (an id).

I can call echo exec("php /var/www/unity/src/emailer.php"); fine, but the moment I add anything like echo exec("php /var/www/unity/src/emailer.php?id=123"); the exec call fails.

How can I do this?