Joomla 2.5 cant use $this in helper file

I have created a component and a plugin in joomla 2.5 and there is a Helper file in the component which will have many useful functions and I plan to call one of its function which then calls another function in the helper by this code:

$this->getinformation();

and it gives me this error :

Fatal error: Call to undefined method

My questions are:

Why can't I call a function in a helper in Joomla?
How can I call a function inside helper class?
Is there any class structure which i missed in this code?

Solution

Helper files are typically called statically and not using $this

First create your helper file and add methods like this:

Class myHelper {

    //This method can call other methods
    public static function myMethod($var) {

        //Call other method inside this class like this:
        self::myOtherMethod($var);

    }

    //This method is called by myMethod()
    public static function myOtherMethod($var) {

        //Put some code here

    }

}

Simply include the helper file like this in the documents that you would to use it:

require_once JPATH_COMPONENT.'/helpers/my_helper.php';

Then use it like this:

myHelper::myMethod($var);

myHelper::myOtherMethod($var);