<?php
    $this->db->select('fname,lname');
    $this->db->from('users');
    $this->db->group_by('lname');
    $this->db->order_by('fname');
    $query= $this->db->get();
    $result = $query->result_array();
    foreach($result as $row)
    {
        $final_arr[] = $row['fname'].' '.$row['lname'];
    }
    $json = json_encode($final_arr);
    echo $json;
?>

["Ajeet Jaiswal","Ankit Gupta","Ankit Tandon","Arun  Sharma","ayush adsaeea","ayush tripathi","Khursheed Abidin","Nishant Chaudhary","Nitin Vishnoi","Praveen Kumar","Priyank  Agrawal","Rahul  ahluwalia","RAZA IMAM","salman saifi","Somesh Anand","Sunil Singh"]