用于sql查询的php函数不起作用--500错误

Question

我的代码无效......它只返回500错误:(

function getfromdb($select, $tablenme, $wherewhat, $whereval) { 

  $msntps = $conn->query("SELECT $select FROM $tablenme WHERE $wherewhat = '$whereval'")->fetch_object()->$selectval; 
  return $msntps;
}

然后我希望得到一些

echo getfromdb("email", "users", "username", "testusername");