Is there a proper way to document a constant defined using define()? @var doesn't really make sense. The only thing I can think of is to omit the tag, and just write the description in the PHPdoc comment.
phpDocumentor does not recognize or utilize a @const tag. phpDocumentor recognizes a constant when it sees the "define" keyword in the code. Its output templates will show all constants in the output documentation, listed as constants. The only thing needed in the constant's docblock is a description, although many other "standard" tags are allowed if you feel like you need them [1].