Warning: Undefined variable $sql in C:\xampp\htdocs\header.php on line 139

Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql) must be of type mysqli, null given in C:\xampp\htdocs\header.php:139 Stack trace: #0 C:\xampp\htdocs\header.php(139): mysqli_query(NULL, 'SELECT * FROM s...') #1 C:\xampp\htdocs\index.php(9): include('C:\\xampp\\htdocs...') #2 {main} thrown in C:\xampp\htdocs\header.php on line 139