The PHP manual says
Like static members, constant values can not be accessed from an instance of the object.
which explains wh
To quote the manual:
As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).
It goes on to use this example:
$class = new MyClass();
echo $class::constant."\n"; // As of PHP 5.3.0
So $inst::someconstant is supposed to work.
As to why $this->inst::someconstant gives a parsing error, I don't know. PHP is funny about some things.