[fw-general] Change property visibility in subclass?

[Tony Ford]

I understand it make no sense to change a private member to protected or 
a protected member to public, but what about the other way around? Example:

class Foo
{
public $my_prop = '';
}
class Bar extends Foo
{
protected $my_prop = 'bleh';
}

Why can't I do this in php? I can't think of a reason you "should not" 
be allowed to do this.

Thanks,
Tony