Using $this when not in object context even when I am not using a static class

Question

I am trying to develop a calendar module which has the following class that prints the calendar

    class modJeventsCalHelper
    {
     public function showcal() {
        $year  == null;

        $month == null;

        if(null==$year&&isset($_GET['year'])){

            $year = $_GET['year'];

        }else if(null==$year){

            $year = date("Y",time());  

        }          

        if(null==$month&&isset($_GET['month'])){

            $month = $_GET['month'];

        }else if(null==$month){

            $month = date("m",time());

        }                  

        $this->currentYear=$year;

        $this->currentMonth=$month;

        $this->daysInMonth=$this->_daysInMonth($month,$year);  

        $content='<div id="calendar">'.
                        '<div class="box">'.
                        $this->_createNavi().
                        '</div>'.
                        '<div class="box-content">'.
                                '<ul class="label">'.$this->_createLabels().'</ul>';   
                                $content.='<div class="clear"></div>';     
                                $content.='<ul class="dates">';    

                                $weeksInMonth = $this->_weeksInMonth($month,$year);
                                // Create weeks in a month
                                for( $i=0; $i<$weeksInMonth; $i++ ){

                                    //Create days in a week
                                    for($j=1;$j<=7;$j++){
                                        $content.=$this->_showDay($i*7+$j);
                                    }
                                }

                                $content.='</ul>';

                                $content.='<div class="clear"></div>';     

                        $content.='</div>';

        $content.='</div>';
        return $content;   
    }
}

I have then stored the content in $calendar

$calendar = modeventscrollerHelper::showcal();

However I get an error

Using $this when not in object context

I have placed a test class to print hello

   public static function getscroller($params)
    {
        return 'Hello, World!';
    }

Which works just fine when I use

$scroller = modeventscrollerHelper::getscroller($params);

Could you please help me resolve this?

Thanks, Sai.