print message if $something is supplied instead of another variable

[nubs]

i want, if the variable $whatever is supplied instead of another one (like a checklist for example), to print the message "invalid argument". how do i do this? just theoretically.

is it

<?php if ($whatever): print "invalid argument"; ?>

[bradgrafelman]

[man]isset/man?

Not sure what else to suggest - perhaps you should show us some example code of the scenario you're describing, as I don't know that I understand what you're asking.