Fatal error: Uncaught mysqli_sql_exception: Incorrect integer value: '' for column 'userid' at row 1 in /var/www/html/add_to_cart.php:12 Stack trace: #0 /var/www/html/add_to_cart.php(12): mysqli_query() #1 {main} thrown in /var/www/html/add_to_cart.php on line 12