I *have* heard claims that something like this is preferrable, though:
if (FALSE === $variable)
I believe I read a comment somewhere once that writing your IF statements that way helped to trigger an error message when the coder forgot to use the double equal sign (==) in the statement. I haven't adopted it yet, but I think that's the sole reason for it. Basically you can't make an assignment (=) to a constant or some non-variable target.