How can i declare Mysqli Connection global in my Class?

Question

I try at the Moment to declare my MySQLi Connection in my global Class. But it isn't working?

What am I doing wrong? Does anyone have a code example?

Heres my try

define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', '');
define('DB_DATABASE', '');
$db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);

And in my Class

 function getUserkey($email)
            {
            $db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);
                $query=mysqli_query($db,
                "
                SELECT
                 *
                 FROM 
                 users
                 WHERE
                  email='$email'
                 ");

                while($row=mysqli_fetch_array($query,MYSQLI_ASSOC))
                {
                $user_key=  $row['user_key'];
                }
                return $user_key;
                mysqli_close($db);
            }

I dont know where is the Problem ?

Answer 1

First of all if the first code is in another file then do some change :-

<?php
error_reporting(E_ALL); //check all type of errors
ini_set('display_errors',1); // display those if any happen
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', '');
define('DB_DATABASE', ''); // provide database name otherwise connection will not create
$db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE) or die(mysqli_connect_error()); // add connection error checking code

And in second file:-

<?php
  error_reporting(E_ALL); //check all type of errors
  ini_set('display_errors',1); // display those if any happen
  include_once('first file path');
  function getUserkey($email,$db){
    // $db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE); no need
      $query=mysqli_query($db,"SELECT * FROM users WHERE email='".$email."'") or die(mysqli_error($db));
      while($row=mysqli_fetch_array($query,MYSQLI_ASSOC)){
        $user_key=  $row['user_key'];
      }
      return $user_key;
      mysqli_close($db);
    }
?>