How to pass string variable from dart to php file - php

Question

I'm trying to send string variable to php file with post method this is my function in dart:
Future<Verify> verifyOTP() async {
var response_verifyotp = await http.post(Uri.parse(linkverifyOTP), body: {
"OTP_code": SOTP,
});
if (response_verifyotp.statusCode == 200) {
print(response_verifyotp.body);
st_verfiy = Verify.fromJson(json.decode(response_verifyotp.body));
print(st_verfiy.status);
}
}
but it doesn't pass correctly still appear to me it's missing value
I'm try also with: "OTP_code": SOTP.toString(),
it's same
This is my php code:
<?php
session_start();
$ch = curl_init();
$OTP_code=$_POST["OTP_code"];
if(isset($_SESSION["id"]))
$id=$_SESSION["id"];
curl_setopt($ch, CURLOPT_URL, "https://www.msegat.com/gw/verifyOTPCode.php");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_HEADER, FALSE);
curl_setopt($ch, CURLOPT_POST, TRUE);
$fields = <<<EOT
{
"lang":"EN",
"userName": "Bloom_Ducks",
"apiKey":"f92b62af08ed0be",
"code":"$OTP_code",
"id": "$id" ,
"userSender":"APP"
}
EOT;
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Content-Type: application/json"
));
$response = curl_exec($ch);
$info = curl_getinfo($ch);
curl_close($ch);
$response=json_decode($response,true);
$code=$response["code"];
if($code == 1){
json_encode(array("status" => 1));
}else{
json_encode(array("status" => 0));
}
?>