When I was learning PHP, I read somewhere that you should always use the upper case versions of booleans, TRUE and FALSE, because the "normal" lowercase versions, true and false, weren't "safe" to use. This applies to NULL and null as well.
It's now been many years, and every PHP script I've written uses the uppercase version. Now, though, I am questioning that, as I have seen plenty of PHP written with the lowercase version (i.e. Zend Framework).
Is/Was there ever a reason to use the uppercase version, or is it perfectly OK to use the lowercase?
The official PHP manual says:
To specify a bool literal, use the constants true or false. Both are case-insensitive.
So yeah, true === TRUE and false === FALSE.
Personally, however, I prefer TRUE over true and FALSE over false for readability reasons. It's the same reason for my preference on using OR over or or ||, and on using AND over and or &&.
The PSR-2 standard requires true, false and null to be in lower case.