docopt/docopt.php — pull request #6 by yriveiro (2013-08-17 19:23:28+00:00)
Title: Allow 0 as default value.

If the default value defined in a option is 0, the parser transform the value to null.

As the default value in the parse function signature is $value = false, when we do the validation !$value , internally PHP resolves '0' as false and this is not correct, 0 can be a valid default value.