在Android开发中，当与PHP后端交互时，可能会遇到“undefined index”错误，这通常是由于未正确获取或解析返回的JSON数据所导致的。以下是解决该问题的代码示例：

import android.os.AsyncTask;
import android.util.Log;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class PHPConnection extends AsyncTask {

    private static final String TAG = "PHPConnection";

    @Override
    protected String doInBackground(String... params) {
        String urlString = params[0];
        String jsonInputString = params[1];
        String result = "";

        try {
            URL url = new URL(urlString);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("POST");
            connection.setDoOutput(true);
            connection.setDoInput(true);
            connection.setRequestProperty("Content-Type", "application/json;charset=utf-8");

            OutputStream outputStream = connection.getOutputStream();
            outputStream.write(jsonInputString.getBytes("UTF-8"));
            outputStream.close();

            int responseCode = connection.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_OK) {
                InputStream inputStream = connection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
                String line;
                StringBuilder stringBuilder = new StringBuilder();

                while ((line = bufferedReader.readLine()) != null) {
                    stringBuilder.append(line);
                }

                result = stringBuilder.toString();
                bufferedReader.close();
                inputStream.close();
            } else {
                result = "Error: " + responseCode;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        return result;
    }

    @Override
    protected void onPostExecute(String result) {
        try {
            JSONObject jsonObject = new JSONObject(result);
            String value = jsonObject.getString("key"); // 替换"key"为你期望获取的键名
            // 处理获取到的数据
        } catch (JSONException e) {
            e.printStackTrace();
            Log.e(TAG, "JSONException: " + e.getMessage());
        }
    }
}

在调用PHPConnection类的地方，传递URL和JSON字符串作为参数。确保替换“key”为你期望获取的键名。通过检查返回的JSON数据是否存在该键名，可以避免“undefined index”错误。