Static method get - is this bad practice?
This isn't horrible persay. It's an implementation of a Factory Method design pattern. It's not bad at all in principle.
However, in your specific example, it's not really doing anything significant, so I'm not so sure if it's necessary. You could eliminate the need by taking a (perhaps optional) parameter to the constructor for the id. Then anyone could call $foo = new Person($id); rather than needing an explicit factory.
But if the instantiation is complex, or you want the ability to build several different people types that can only be determined by logic, a factory method may work better. For example, let's say you need to determine the type of person to instantiate by some parameter. Then, a factory method on Person would be appropriate. The method would determine what "type" to load, and then instantiate that class.
Statics in general are hard to test and don't allow for polymorphic changes like an instance would. They also create hard dependencies between classes in the code. They are not horrible, but you should really think about it if you want to use one. An option would be to use a Builder or a Abstract Factory. That way, you create an instance of the builder/factory, and then let that instance determine how to instantiate the resulting class...
One other note. I would rename that method from Person::get() to something a little more semantically appropriate. Perhaps Person::getInstance() or something else appropriate.
This blog post should tell you why people don't like static methods better than i could:
http://kore-nordmann.de/blog/0103_static_considered_harmful.html
The question that strikes me most about your current code snippet: Is a Person allowed to NOT have an Id ?
I feel like that should be an constructor argument if it's representing a real Person. If you use that class to create new persons that ofc might not work.
The difference between the 2 calls is minor. Both "create" a Person class and set the Id so you are not winning / loosing anything there when it comes to 'hard wired dependencies'.
The advantage only shows when you want to be able to pass a Person into another object and that objects needs to change the ID (as an example, the blog post should explain that better than i did here).