CI：非法字符串偏移警告在php 7.1或更高版本中

I work on CI 3.1.7 php 7.0 and when i upgrade my php version to 7.1.13 and 7.2 i got this error in Codeigniter.

Note: it will work fine in php 7.0.

controller code:

class Site extends CI_Controller {
public $data="";
function __construct(){
    parent::__construct();
    //var_dump($this->site_model->get_web_data(1));die();
    $this->data["C_mobile_number"]=$this->site_model->get_web_data(1);

model code

function get_web_data($id){

    $result=$this->db->where('drid',$id)
                ->get('web_data');
    if($this->db->affected_rows() > 0){
        $data=$result->result();    
        return $data[0]->data;
    }else{
        return "";
    }


}

var_dump result : E:\wamp64\www\**\application\controllers\Site.php:9:string '+91.**********' (length=14)

In php 7.1 or higher i got this error

Severity: Warning

Message: Illegal string offset 'C_mobile_number'

Filename: controllers/Site.php Line Number: 10

Any salution?

Before $this->data["C_mobile_number"]=... insert $this->data = [];. This is initiated $this->data as array;