而不是数据库中使用dbname

in your db connection string instead of database use dbname

  $conn = new PDO('mysql:host=localhost;database=evouchers', $user, $password);
  ---------------------------------------^



 $conn = new PDO('mysql:host=localhost;dbname=evouchers', $user, $password);

文档链接: http://php.net/manual/en/pdo.connections .php