gistfile1.php

Class IpRange
{
    public function setStartingAddress($startingAddress)
    {
        InputValidator::assertValidAddress($startingAddress);

        $this->startingAddress = (int) $startingAddress;
        return $this;
    }
}

I think you are overstating the value of a mock here. YAGNI, right? With PHP 5.4 I would have used a trait...

I don't see how a trait would make any difference; a trait adds the same testing issues as a static, global function, singleton or any other piece of global state

Let's say I ditch the "staticness" of the InputValidator class... without dependency injection, how would I be able to check if the method was invoked?

That depends on how you make your validation methods visible; when using extension you can Stub your fixture as shown in the previous example. As a trait? You can't. Traits are effectively global state as well. SOLID has Dependency Inversion as a principle because it allows for things like testability.

Extension is not an option in my case, given multiple inheritance is (unfortunately) not a language feature of PHP.