Custom CSV export model

Can Someone here correct me what's wrong with my code ? I am actually trying to create model instead of adding function in Grocery crud library, Please someone help me, I really don't know how to extend library functions using models.

My model custom_export_model.php

[peter@localhost models]$ pwd
/var/www/html/virtual/Development/DB/application/models

[peter@localhost models]$ cat custom_export_model.php
    <?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
     
    class Custom_export_model extends grocery_CRUD_model {
     
    	function __construct() { parent::__construct(); }
  

	public function exportToCSV($state_info = null)
	{

		$data = $this->get_common_data();

		$data->order_by 	= $this->order_by;
		$data->types 		= $this->get_field_types();

		$data->list = $this->get_list();
		$data->list = $this->change_list($data->list , $data->types);
		$data->list = $this->change_list_add_actions($data->list);

		$data->total_results = $this->get_total_results();

		$data->columns 				= $this->get_columns();
		$data->primary_key 			= $this->get_primary_key();

		@ob_end_clean();

		$this->_export_to_csv($data);

	}


	function _export_to_csv($data)
	{
		
		$string_to_export = "";
		foreach($data->columns as $column){
			$string_to_export .= '"'.$column->display_as.'"'.",";
		}
		$string_to_export .= "\n";

		foreach($data->list as $num_row => $row){
			foreach($data->columns as $column){
				$string_to_export .= '"'.$this->_trim_export_string($row->{$column->field_name}).'"'.",";
			}
			$string_to_export .= "\n";
		}

		// Convert to UTF-16LE and Prepend BOM
		$string_to_export = "\xFF\xFE" .mb_convert_encoding($string_to_export, 'UTF-16LE', 'UTF-8');

		$filename = "export-".date("Y-m-d_H:i:s").".csv";

		header('Content-type: application/vnd.ms-excel;charset=UTF-16LE');
		header('Content-Disposition: attachment; filename='.$filename);
		header("Cache-Control: no-cache");
		echo $string_to_export;
		die();
	}


    }

My controller

Before calling load view and render function, I am doing this

		$crud->set_subject('testdata');

		$state = $crud->getState();
  		$state_info = $crud->getStateInfo();
  		if($state == 'export')
  		{
			$crud->set_model('custom_export_model');
			$crud->basic_model->exportToCSV($state_info);
  		}

		$output = $crud->render();
		$this->_example_output($output);

Thank you all

Peter

Peter