Warning: Undefined variable $app_con in C:\xampp\htdocs\forms\gallery_data.php on line 4

Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql) must be of type mysqli, null given in C:\xampp\htdocs\forms\gallery_data.php:4 Stack trace: #0 C:\xampp\htdocs\forms\gallery_data.php(4): mysqli_query(NULL, 'SELECT * FROM w...') #1 {main} thrown in C:\xampp\htdocs\forms\gallery_data.php on line 4