Fatal error: Uncaught Error: Call to a member function query() on null

I know I have the error because of the scope in my method and the variable $conn is not defined there. How do I define that protected variable in my second class so I could make connection to my DB and fetch array.

[code] class Config{
private $host;
private $user;
private $pass;
private $db;
protected $conn;

// open connection to database
public function __construct() {
$this->db_connect();
}

static public function db_connect(){
$host = ”;
$user = ”;
$pass = ”;
$db = ”;

$conn = new mysqli($host, $user, $pass, $db);
if(mysqli_connect_errno()){
$msg = “Database connection failed: “;
$msg .= $conn->connect_error;
$msg .= ” (” . $conn->connect_errno . “)”;
exit($msg);
}
}
}[/code]

My second class

[code]class Database extends Config{

public function __construct(){
parent::__construct();
}

static public function find_by_sql($sql){
$result = $conn->query($sql);
if(!$result){
exit(“Database query failed.”);
}

//gather result into objects
$object_arr = [];
while($record = $result->fetch_assoc()){
$object_arr[] = htmlentities($record);
}
// result object method to free result set
$result->free();
}

}[/code]

now the use

[code]Database::db_connect();
$sql = “SELECT * FROM Events “;
$event = Database::find_by_sql($sql);

echo ‘<pre>’;
echo print_r($event);
echo ‘</pre>’;[/code]

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