php插入数据库的一个踩坑点

    $Myaccount='ad';
    $MyPwd='ad';

   $reg_sql="INSERT INTO user VALUES ($uid,$Myaccount,$MyPwd)";
  //$reg_sql="INSERT INTO user  VALUES(6688,'ppppppp','da') ";
   $state=mysqli_query($conn,$reg_sql,MYSQLI_STORE_RESULT);
   var_dump($state);
   $error=mysqli_error($conn);
   echo "错误描述->".$error;
   mysqli_close($conn);

这么写，会导致无法插入数据库
报错：Unknown column 'ad' in 'field list'
必须要给变量加上单引号

  $uid=3;
    $Myaccount='ad';
    $MyPwd='ad';

   $reg_sql="INSERT INTO user VALUES ($uid,'$Myaccount','$MyPwd')";
  //$reg_sql="INSERT INTO user  VALUES(6688,'ppppppp','da') ";
   $state=mysqli_query($conn,$reg_sql,MYSQLI_STORE_RESULT);
   var_dump($state);
   $error=mysqli_error($conn);
   echo "错误描述->".$error;
   mysqli_close($conn);