This might make it a bit more obvious (or not?)
class classname{
public $message = 'Hello, World!';
private $nope = "Can't touch this";
}
$foo = new classname();
echo $foo->message . "\n";
$foo->message = 'Goodbye, cruel world.';
echo $foo->message . "\n";
echo $foo->nope;
Result:
Hello, World!
Goodbye, cruel world.
PHP Warning: Uncaught Error: Cannot access private property classname::$nope . . .