Accessing Private Function From Outside Class

I'm learning OO stuff, and came across this:

class n{

    private function f($v){
        return $v*7;
    }

    function c(){
       return $this->f(5);
    }
}

$o = new n;
echo $o->c(); //returns 35

Doesn't that beat the purpose of declaring functions private if I can access it still from outside the class? Shouldn't this be blocked altogether? Am I missing something? Please help clear up. Thanks

Solution

Public functions are meant to perform operations on an instance of that class. Say, Save().

The internal workings of Save() are not interesting for the caller; he simply wants to save it and doesn't care how that happens.

As a matter of style, you might or might not want to actually perform the saving in that method. It might depend on design choices, or on properties of the object. See:

class FooObject
{

    private $_source;

    public function Save()
    {

        if ($this->_source == "textfile")
        {
            $this->saveToTextfile();
        }
        elseif ($this->_source == "database")
        {
            $this->saveToDatabase();
        }
    }

    private function saveToTextfile()
    {
        // Magic
    }

    private function saveToDatabase()
    {
        // Magic
    }
}

You don't want anyone to call the private methods directly, because they are for internal use only. However, a public method may indirectly call a private method.