为什么PHP中的$ _SERVER ['SERVER_NAME']代码给我错误？

Question

我是编程新手，因此决定尝试PHP。 我正在尝试设置variable $_SERVER["SERVER_NAME"]。 不幸的是，我遇到一些错误。

该代码是从this Youtube tutorial复制而来的，因此应该是正确的。

$server = [
         'Host Server Name' => $_SERVER['SERVER_NAME'],
         'Host Header' => $_SERVER['HTTP_HOST'],
         'Server Software' => $_SERVER['SERVER_SOFTWARE'],
         'Document Root' => $_SERVER['DOCUMENT_ROOT'],
         'Current Page' => $_SERVER['PHP_SELF'],
         'Script Name' => $_SERVER['SCRIPT_NAME'],
         'Absolute Path' => $_SERVER['SCRIPT_FILENAME']
];


//print_r($server);

// Create Client Array
$client = [
    'Client System info' => $_SERVER['HTTP_USER_AGENT'],
    'Client IP' => $_SERVER['REMOTE_ADDR'],
    'Remote Port' => $_SERVER['REMOTE_PORT']
];

错误：

Notice: Undefined variable:** server in C:\xampp\htdocs\phpsandbox\website2\server-info.php on line 7

Fatal error: Uncaught Error: Unsupported operand types in C:\xampp\htdocs\phpsandbox\website2\server-info.php:7 Stack trace: #0 {main} thrown in C:\xampp\htdocs\phpsandbox\website2\server-info.php on line 7

感谢任何人的支持！