php - Unable to view property inherited from parent class

As you said, the $Balance property is private. Private means it is not accessible from outside the defining class, not even from child classes that inherit the property.

class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

If we run this code we get the error:

baz
PHP Notice:  Undefined property: Bar::$bar in php shell code on line 2

So, you have two choices. You can use your getter instead:

class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->getBar();}
}
new Foo;
new Bar;

Or, you can declare the property as protected instead of private:

class Foo {
    protected $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

Either of these give the expected output:

baz
baz

See documentation here for more details on property and method visibility.