Warning: Undefined variable $conn in /var/www/html/navbar.php on line 4

Fatal error: Uncaught TypeError: mysqli_real_escape_string(): Argument #1 ($mysql) must be of type mysqli, null given in /var/www/html/navbar.php:4 Stack trace: #0 /var/www/html/navbar.php(4): mysqli_real_escape_string() #1 /var/www/html/forgot-password.php(19): include('...') #2 {main} thrown in /var/www/html/navbar.php on line 4