mysqli_query採坑記錄

使用PHP向mysql插入數據的時候，一直沒插入成功：

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

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

echo br;

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

在數據庫中查詢數據一直查詢不到，是因爲username和password沒用加上引號：

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