Google geolocation using memory information Curl 400 Bad request PHP

Question

I am trying to get latitude and longitude from cell information using Google geolocation api . It requires valid JSON with information such as MCC, MNC, cellId, lac, etc .. My request to send PHP is as follows.

<?php
header("Access-Control-Allow-Origin: *");

$mcc = $_POST["mcc"];
$mnc = $_POST["mnc"];
$cellId = $_POST["cellId"];
$lac = $_POST["lac"];

$post_array = array(
                "cellId" => (int) $cellId,
                "locationAreaCode" => (int) $lac,
                "mobileCountryCode" => (int) $mcc,
                "mobileNetworkCode" => (int) $mnc,
            );

$post_data = json_encode(array('cellTowers' => array($post_array)));

echo $post_data;


$url = "https://www.googleapis.com/geolocation/v1/geolocate?key=".$api_key; // not including api key here but its there in my code

$ch = curl_init();
curl_setopt_array($ch, array(
    CURLOPT_URL => $url,
    CURLOPT_POST => true,
    CURLOPT_HEADER => false,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_SSL_VERIFYPEER => false,
    CURLOPT_SSL_VERIFYHOST => 2,
    CURLOPT_HTTPHEADER => array(
        'Content-Type: application/json'
    ),
    CURLOPT_POSTFIELDS => $post_data
));
$result = curl_exec($ch);

echo "Result: ".$result;

curl_close($ch);

?>

However, I get an error message in the response. The error is shown below.

Result: {
 "error": {
  "errors": [
   {
    "domain": "geolocation",
    "reason": "invalidRequest",
    "message": "Bad Request"
   }
  ],
  "code": 400,
  "message": "Bad Request"
 }
}

I thought my JSON was not in the correct format, but it worked with the following command line execution, so this may not be a problem.

$ curl -d @your_filename.json -H "Content-Type: application/json" -i "https://www.googleapis.com/geolocation/v1/geolocate?key=API_KEY"

The above command in the terminal gives the correct wavelength and longitude with the same JSON in the file. What am I doing wrong?