Why are PHP booleans both lower case and upper case?

Question

Is there a difference between true and TRUE or false and FALSE in PHP?

Answer 1

If you intend to use JSON then the standard RFC7159 says :

The literal names MUST be lowercase. No other literal names are allowed.

And from Php 5.6 :

json_decode() now rejects non-lowercase variants of the JSON literals true, false and null at all times, as per the JSON specification

And according to PSR-2 standard :

PHP keywords MUST be in lower case.

The PHP constants true, false, and null MUST be in lower case.

Ps.: I could not post link to the RFC7159 because of SO limitations.

Answer 2

Constants are case-sensitive per default. But for symmetry to the other identifier namespaces, they can be defined case-insensitively:

 define("mixedCASE", 123, TRUE);

 print MiXeDcAsE;

And that's just how TRUE and FALSE were pre-declared. (They aren't parser/language builtins.)