How to implement abstract properties in PHP?

Question

How do I make $myVar abstract in ParentClass?

<?php
abstract class ParentClass
{
    public $myVar;  //Make this abstract

    abstract public function get();

    public function someMethod() {}
}

class childClass extends ParentClass
{
    public $myVar = 123;

    public function get(){}

    public function someOtherMethod() {}
}