Hello i cant understand what this variables mean havent seen them before

$ser=$_SERVER['HTTP_HOST'];
$ref=$_SERVER['HTTP_REFERER'];

  $url = $_SERVER['SCRIPT_NAME'];
  $host = $_SERVER['SERVER_NAME'];

  $url = $_SERVER['SCRIPT_FILENAME'];

                                                             $ura = $_SERVER['SCRIPT_NAME'];
                                                             $host = $_SERVER['SERVER_NAME'];

this script connects with its own database but i want it to connect to a database that i have already.
i have include my db.php file so it does connect with my database and created a required table on my database

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