mysqli_real_escape_string函数

Question

我正在尝试创建一个缩短的mysqli_real_escape_string函数

代码在直接使用时有效，但是在尝试转换为函数时会出错

这很完美： '''

$conn = new mysqli($dbservername, $dbusername, $dbpassword, $dbname);
$username = $conn->real_escape_string($_GET['user_name']);

''' 但是使用以下代码： '''

$conn = new mysqli($dbservername, $dbusername, $dbpassword, $dbname);
function cleanstr($cleanval){
    $cleanval= $conn->real_escape_string($cleanval);
    //$cleanval =mysqli_real_escape_string($conn, $cleanval);
    return $cleanval;
}
$username = cleanstr($_GET['user_name']);

''' 我收到错误消息：

........................

( ! ) Notice: Undefined variable: conn in C:\wamp64\www\Data\reg\functions.php on line 21
Call Stack
#   Time    Memory  Function    Location
1   0.0004  403064  {main}( )   ...\verify_email.php:0
2   0.1575  420064  cleanstr( ) ...\verify_email.php:10

( ! ) Fatal error: Uncaught Error: Call to a member function real_escape_string() on null in C:\wamp64\www\Data\reg\functions.php on line 21
( ! ) Error: Call to a member function real_escape_string() on null in C:\wamp64\www\Data\reg\functions.php on line 21
Call Stack
#   Time    Memory  Function    Location
1   0.0004  403064  {main}( )   ...\verify_email.php:0
2   0.1575  420064  cleanstr( ) ...\verify_email.php:10

................................................... 我是在做错什么，还是没有办法在自定义函数中使用real_escape_string