<?php
$result = $this->db->get_where('table',array('mainpage'=>$mp));
$data = array();
$out = array();

foreach($result->result() as $row){
        $dv = json_decode($row->subpage);

        $flat = array();

        foreach ( $dv as $item ) {
            $flat = array_merge( $flat, explode( ',', $item ) );
        }

        $out = array_merge( $out, $flat );
}
print_r($out); // and my output is as: array([1]=>11 [2]=>22 [3]=>13 ...)
return $out;

Fatal error: Using $this when not in object context on line 2