What are the names of the properties of a class that can be used in zephyr?

I'm creating an extension containing the class

namespace ExtABC;

class ClassA
{
    public $A1;

    public function __construct()
    {
    }
}

During compilation I get an error

Syntax error in extabc/classb on line 5

    public $A1;
------------^

Why the simple property name $A1 can not be used in zephyr?

Property name $A1 is not essential for me, I just wanted to use it in my previous post in this forum. This name has caused an error and instead it I used the name of the property $abc )))

Let's look at another example. Let extension ExtABC contains two classes ClassA and ClassB.

namespace ExtABC;

class ClassA
{
    protected $id;
    protected $name;

    public function __construct()
    {
        let $this->id = "";
        let $this->name = "";
    }

    public function __get(var $prop)
    {
        switch ($prop)
        {
            case "ID":
                return $this->id;
            case "Name":
                return $this->name;
        }
    }

    public function __set(var $prop, var $value)
    {
        switch ($prop)
        {
            case "ID":
                let $this->id = $value;
                break;
            case "Name":
                let $this->name = $value;
                break;
        }
    }
}

namespace ExtABC;
use ExtABC\ClassA;

class ClassB
{
    public function __construct()
    {
        var $objectA;
        let $objectA = new \ExtABC\ClassA();
        let $objectA->ID = "objectA";
        let $objectA->Name = "objectA";
    }
}

During compilation I get an error

Syntax error in extabc/classb on line 10

        let $objectA->ID = "objectA";
----------------------^

This already is a very significant error.

So what are the names of class properties and class methods that are acceptable in zephir?