The 'defaultText' field inside the 'info' array of a ReflectionParameter
instance is matched against a regex that checks if it complies with the
accepted format for labels and constants names.

Then it checks whether or not it is one of the following reserved words:
'NULL', 'false' and 'true'. If it is, then the function returns FALSE.
Otherwise, TRUE.

…rameter class.

The method getDefaultValueConstantName returns NULL in case the
value is NOT a constant; otherwise, it returns the constant's name.

To check if the value is indeed a constant, we use the method
isDefaultValueConstant method.

Correct the doc on return values to lowercase booleans for the
isDefaultValueConstant method.
Add return of NULL (to the method cited above) when the given argument does not
have a default value.