我正在尝试构建一个简单的自定义CMS，但出现错误：

警告：mysqli_query（）期望参数1为MySQLi，在中为null

为什么会出现此错误？我所有的代码已经是MySQLi，我正在使用两个参数，而不是一个。

$con=mysqli_connect("localhost","xxxx","xxxx","xxxxx");

//check connection
if (mysqli_connect_errno($con))
{
    echo "Failed to connect to MySQL:" . mysqli_connect_error();
}

function getPosts() {
    $query = mysqli_query($con,"SELECT * FROM Blog");
    while($row = mysqli_fetch_array($query))
    {
        echo "<div class=\"blogsnippet\">";
        echo "<h4>" . $row['Title'] . "</h4>" . $row['SubHeading'];
        echo "</div>";
    }
}