The latter choice would be the correct one. A boolean is a predefined constant in PHP, so it must not be encased inside any sort of quotation marks. The former choice has a value of “FALSE”, but it’s cast as a string, and so you’ll have to cater for that when wanting to see if the function returns FALSE or not.
will output “FALSE is true”, so “FALSE” is not really false; false is, so use the second option.
No need to put it all in caps though (but you can if you want to).