Plzz tell me about this

Notice: Undefined variable: connect in C:\xampp\htdocs\stock\php_action\db_connect.php on line 10

Notice: Trying to get property of non-object in C:\xampp\htdocs\stock\php_action\db_connect.php on line 10
Successfully connected
Notice: Undefined variable: connect in C:\xampp\htdocs\stock\index.php on line 26

Fatal error: Call to a member function query() on a non-object in C:\xampp\htdocs\stock\index.php on line 26

<?php
 $db_name = "stock";
 $mysql_user = "root";
 $mysql_pass = "";
 $server_name = "localhost";
 $con = mysqli_connect($server_name,$mysql_user,$mysql_pass,$db_name);


//check connection
if($connect->connect_error) {
  die("Connection Failed  " . $connect->connect_error);
} else {
   echo "Successfully connected";
}

?>

<?php

require_once 'php_action/db_connect.php';

session_start();

if (isset($_SESSION['userId'])) {
	header('location: http://localhost/stock/dashboard.php');
}

$errors = array();

if ($_POST) {
	$username = $_POST['username'];
	$password = $_POST['password'];

	if (empty($username) || empty($password)) {
	if ($username == "") {
		$errors[] = "Username is required";
			}		
			if ($password == "") {
				$errors[] = "Password is required";
			}
	}else {
		$sql = "SELECT * FROM users WHERE username == '$username'";
		$result = $connect->query($sql);

		if($result->num_rows == 1) {
			$password == md5($password);
			// exists
			$mainSql = "SELECT * FROM users WHERE username == '$username' AND password == '$password'";
			$mainResult = $connect->query($mainSql);

			if($mainResult->num_rows == 1) {
				$value = $mainResult->fetch_assoc();
				$user_id = $value['user_id'];

				// set session
				$_SESSION['userId'] = $user_id;

				header('location: http://localhost/stock/dashboard.php');
	}else{
		$errors[] = "Incorrect username or password combination";
	}
}else{
	$errors[] = "Username doesnot exists";