mysqli_query采坑记录

使用PHP向mysql插入数据的时候，一直没插入成功：

$username = $_POST['username'];
$password = $_POST['password'];

$connect = mysqli_connect("localhost", "root", "123456", "test", 3306);
echo "
";
var_dump($connect);
echo "
";

echo br;

if (!mysqli_errno($connect)) {
    echo $username . "-----" . $password;
    mysqli_query($connect, "insert into user_info(username,password) values ($username, $password);"); 
}

在数据库中查询数据一直查询不到，是因为username和password没用加上引号：

  mysqli_query($connect, "insert into user_info(username,password) values('$username', '$password');");