Optional

Validates if the given input is optional or not. By optional you may interpret as null or an empty string ('').

v::optional(v::alpha())->validate(''); // true
v::optional(v::digit())->validate(null); // true

Also you can defined what values you want as optional values:

v::optional(v::alpha(), array(null))->validate(''); // false

The example bellow returns false because only null is accepted as optional value, and '' is not.