My code is:
classes/presenter/index.php
class Presenter_Index extends Presenter
{

    public function view()
    {
         $this->title = 'Testing this Presenter thing';
         $this->echo_upper = function($string)
                            {
                                echo strtoupper($string);
                               
                            };
   }
}

views/index.php
<h1><?php echo $title; ?></h1>
<?php
    $echo_upper('this string');
?>

This error shows:

Fatal Error!

ErrorException [ Fatal Error ]:
Function name must be a string

APPPATH/views/index.php @ line 3

Thanks,

Hopping to solve this problem soon.