function search_prd()
           {
             $query=$this->input->post();
             $this->load->model('prd/addprd');
             $prd_search=$this->addprd->prd_search($query);
             $this->load->view('prd_search',['prd_search'=>$prd_search]);
           }

function prd_search($query)
       {
         $q= $this->db->from('purchase')
            ->like('item_name',$query)
            ->get();
            return $q->num_rows();
        }

<input name="search_value">

$query = $this->input->post('search_value'); //notate the field being used here

$this->db->like('column', $query); //query is now a string

$this->db->where_in('column', $query); //if you assign $query to $this->input->post();