I forgot to state why exactly what I think the problem is.
As you can see, the parameters are determined to be either valid or invalid, and are then added to the appropriate static array. After all inputs have been validated and none are determined to be erroneous, the static array $valid is used to pass information onto other classes and the database, like:
if (empty(Validate::$errors))
{
CreateSubscription(Validate::$valid['email']);
}
So can the data be corrupted by the time it gets to that point, say, if a third or fourth user were to submit data to the Validate class just before the other user's CreateSubscription method were called?