Kodunda hatalar var. Bu şekilde dene;
$username = $_POST['username'];
$password = password_hash($_POST['password'], PASSWORD_DEFAULT);
$sql = "INSERT INTO users (`username`,`password`,`pre`,`bakiye`,`uyelik`,`gi`,`ai`) VALUES (?,?,0,5,'Baslangic',50,300)";
$stmt = $con->prepare($sql)...