Uppercase Booleans vs. Lowercase in PHP

[Aspect]

When I was learning PHP, I read somewhere that you should always use the upper case versions of booleans, \[code\]TRUE\[/code\] and \[code\]FALSE\[/code\], because the "normal" lowercase versions, \[code\]true\[/code\] and \[code\]false\[/code\], weren't "safe" to use.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?edit: Forgot to mention that this applies to \[code\]NULL\[/code\] and \[code\]null\[/code\] as well.