Passing Variables to an iframe

This may be more a general HTML question, so let me know if I should post somewhere else. I was hoping to do some logic in a script, and then pass the results of that script to an iframe for more processing. Is it secure to include those variables as get parameters to the iframe, though? In other words, if I have something like this: <iframe src="http://example.com/?accesseverything=true"> where I use PHP to generate the src for the iframe. Could someone just use Firebug or something to set that variable? Is there a better way of passing it instead? Thanks, Waynn