The Moodle Coding Style (MCS) states:

Constants should always be in upper case, and always start with[ Frankenstyle|https://moodledev.io/general/development/policies/codingstyle/frankenstyle] prefix and plugin name (in case of activities the module name only for legacy reasons). They should have words separated by underscores.

There seems to have been a change between PSR-1 and PSR 12

PSR-1 states:

Class constants MUST be declared in all uppercase with underscore separators

PSR-12 states:

Doesn’t make specific mention, however the examples are in lowercase.

Required decision:

Providing we’re correct and it’s PSR 12, should we adopt camel cased constants?

MCS: https://moodledev.io/general/development/policies/codingstyle#constants 

PSR-1: https://www.php-fig.org/psr/psr-1/#41-constants 

PSR-12: https://www.php-fig.org/psr/psr-12/#43-properties-and-constants