React Native Fetch API returning text with escape characters

I am a developing a React Native app and fetching API for login.Below is my store where I an calling API.

auth.js:

import AsyncStorage from '@react-native-community/async-storage';

export const LOGIN ='LOGIN';

export const login=(textemailid,textpassword) =>{
formData.append('txtUemail',textemailid);
formData.append('txtUpass',textpassword);
return async dispatch =>{
 await fetch('https://----------------------/login.php',
{
  method:'post',
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
},
  body: formData
})
.then(res => res.text())
.then(
  (loginresult) =>{
   console.log(loginresult);
  }).catch(err =>{
    console.log(err);
  })
  const saveDataToStorage = (loginresult) =>{
   
    AsyncStorage.setItem('userData',JSON.stringify({
      loginresult:loginresult
    }))
    }      
    dispatch({type:LOGIN});
    }
    }

I am getting the output in console as in below screen.It gives some spaces and then success.

If I parse the result using json like .then(res => res.json()) it gives

JSON Parse error: Unexpected identifier "success".So I am parsing as .then(res => res.text()) .

In php program I am echoing string as given below:

    if($check == true){
        echo "success";
    }
    else
    {
        echo "Login Failed";  
    }

I want string so that I can allow user to login.I dont know why I am getting escape characters in the result.Can anyone tell me where I am going wrong.Thanks in Advance.

source https://stackoverflow.com/questions/69743628/react-native-fetch-api-returning-text-with-escape-characters