打开指定数据库失败，但是test数据库可以
$con=mysqli_connect('localhost') or die("数据库连接失败".mysqli_error($con));
mysqli_query($con,"set names utf8");
$conn=mysqli_select_db($con,'quanta_food') or die("指定数据库打开失败");
$con=mysqli_connect('localhost') or die("数据库连接失败".mysqli_error($con));
mysqli_query($con,"set names utf8");
$conn=mysqli_select_db($con,'quanta_food') or die("指定数据库打开失败");