Executing mysqli_query inside a function

That’s a variable scope problem. You need to pass $conn to getSiteName():

function getSiteName($con) {
    $row = $con->query("SELECT * FROM siteinfo")->fetch_array();
    return $row["siteName"];
}

$name = getSiteName($con);

Or using a class with constructor injection:

class MyThing
{
    protected $con;

    public function __construct($con) {
        $this->con = $con;
    }

    public function getSiteName() {
        $row = $this->con->query("SELECT * FROM siteinfo")->fetch_array();
        return $row["siteName"];
    }
}

$obj = new MyThing($con);
$name = $obj->getSiteName();

Throughout the class you can use $this->con to access the connection.