php 文件：

$email=$_POST['email'];

    $passworda=$_POST['passworda'];

$sql="SELECT * FROM user WHERE email='".$email."'AND passworda='".$password."' ";
$result=mysqli_query($db,$sql);
$count=mysqli_num_rows($result);
if($count>=1){
echo json_encode("success");
}
else
{
echo json_encode("error");
}

登录页面 Flutter：

class Login extends StatelessWidget {
  TextEditingController email = TextEditingController();
  TextEditingController password = TextEditingController();

  Future login(BuildContext cont) async {
    if (email.text == "" || password.text == "") {
      Fluttertoast.showToast(
        msg: "please complete!",
        toastLength: Toast.LENGTH_SHORT,
        gravity: ToastGravity.CENTER,
        fontSize: 16.0,
      );
    } else {
      var url = "http://192.168.43.150/v1_flutter/lib/php/connection.php";
      var response = await http.post(Uri.parse(url), body: {
        "email": email.text,
        "pass": password.text,
      }, headers: {"Accept":"applicarion/json"});
      var data = jsonDecode(response.body);

      if (data == "success") {
        Navigator.pop(cont);
        Navigator.pushNamed(cont, "/registre");
      } else {
        Fluttertoast.showToast(
          msg: "The user and password does not exist!",
          toastLength: Toast.LENGTH_SHORT,
          gravity: ToastGravity.CENTER,
          fontSize: 16.0,
        );
      }
    }}

控制台：

E/flutter (6084): [错误:flutter/lib/ui/ui_dart_state.cc(198)] 未处理的异常：FormatException：意外的字符（位于 字符 1) E/颤振（6084）：

E/颤动​​（6084）：^ E/颤振（6084）： E/颤振（6084）： #0 _ChunkedJsonParser.fail (dart:convert-patch/convert_patch.dart:1383:5) E/颤振（6084）： #1 _ChunkedJsonParser.parseNumber (dart:convert-patch/convert_patch.dart:1250:9) E/颤振（6084）： #2 _ChunkedJsonParser.parse (dart:convert-patch/convert_patch.dart:915:22) E/颤振（6084）： #3 _parseJson (dart:convert-patch/convert_patch.dart:35:10) E/颤振（6084）： #4 JsonDecoder.convert (dart:convert/json.dart:612:36) E/颤振（6084）： #5 JsonCodec.decode (dart:convert/json.dart:216:41) E/颤振（6084）： #6 jsonDecode (dart:convert/json.dart:155:10) E/颤振（6084）： #7 Login.login (package:mes_v1/pages/Authentification/login.dart:25:18) E/颤振（6084）： E/颤振（6084）：