将mysql转换为mysqli - 如何获取超全局连接对象？

// MySQL
$conn = mysql_connect("localhost", "username", "password");
mysql_select_db("database_name", $conn);

// MySQLi
$conn = new mysqli("localhost", "username", "password", "database_name");

// MySQL
$result = mysql_query("SELECT * FROM table_name", $conn);

// MySQLi
$result = $conn->query("SELECT * FROM table_name");

// MySQL
$row = mysql_fetch_assoc($result);

// MySQLi
$row = $result->fetch_assoc();

// MySQL
mysql_close($conn);

// MySQLi
$conn->close();

$GLOBALS['conn'] = new mysqli("localhost", "username", "password", "database_name");