$this->con->prepare not working - php

Question

i have config file for connection but when ever i use $this->con->prepare() or $this->con->query() are not working.
But when i make separate variable for connection its work fine below are example.
this code in not working
private function isEmailexist($email)
{
$stmt = $this->con->prepare("SELECT * FROM devices WHERE email = ?");
$stmt->bind_param("s",$email);
$stmt->execute();
$stmt->store_result();
$rows = $stmt->num_rows;
echo "Rows : ".$rows;
}
This code work
private function isEmailexist($email)
{
$mysqli = new mysqli("localhost", "root", "", "demo1");
$stmt = $mysqli->prepare("SELECT * FROM devices WHERE email = ?");
$stmt->bind_param("s",$email);
$stmt->execute();
$stmt->store_result();
$rows = $stmt->num_rows;
echo "Rows : ".$rows;
}
this is config file
class Config
{
private $con;
function __construct(){}
function connect()
{
define('DB_USERNAME','root');
define('DB_PASSWORD','');
define('DB_NAME','demo1');
define('DB_HOST','localhost');
$con = new mysqli(DB_HOST,DB_USERNAME,DB_PASSWORD,DB_NAME);
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
echo "Connection success...";
return $this->con;
}
}

Answer 1

Your issue is with setting $con inside of connect()
You are using
// $con is only visible within the scope of the connect function
$con = new mysqli(DB_HOST,DB_USERNAME,DB_PASSWORD,DB_NAME);
but it should be
// this will set the config class's $con property so that it is re-usable
$this->con = new mysqli(DB_HOST,DB_USERNAME,DB_PASSWORD,DB_NAME);