Cannot Send SMS

"I have been trying to send an SMS using a PHP function but it's not working. I have tried everything that I know of, but I'm not getting any results. I am using an API SMS method but I'm getting an error message that says 'Undefined variable $ch'. I am not sure if something is missing or not in my code. Below is the code that I am using."

<?php
include_once "./config.php";

$fname = $_POST["fname"];
$lname = $_POST["lname"];
$email = $_POST["email"];
$phone = $_POST["phone"];
$date = $_POST["date"];
$gender = $_POST["gender"];
$address = $_POST["address"];
$hobby = $_POST["hobby"];

$hobbyInString = serialize($hobby);

// Use prepared statement to prevent SQL injection
$sql = "INSERT INTO details (first_name, last_name, email, mobile_num, dob, gender, address, hobbies) VALUES (?, ?, ?, ?, ?, ?, ?, ?)";
$stmt = mysqli_prepare($conn, $sql);
mysqli_stmt_bind_param($stmt, "ssssssss", $fname, $lname, $email, $phone, $date, $gender, $address, $hobbyInString);
$checkResult = mysqli_stmt_execute($stmt);

if ($checkResult) {
    // Successfully entered data into the database

    // Continue with SMS sending
    $access_token = login();

    $post_data_sms = array(
        "campaignName" => "XXXXXXX",
        "mask" => "XXXXXXX",
        "numbers" => $phone, // Assuming $phone contains the recipient's number
        "deliveryReportRequest" => true,
        "content" => "Test message"
    );

    $ch_sms = curl_init('https://bsms.hutch.lk/api/sendsms');
    curl_setopt_array($ch_sms, array(
        CURLOPT_POST => TRUE,
        CURLOPT_RETURNTRANSFER => TRUE,
        CURLOPT_HTTPHEADER => array(
            'Content-Type: application/json',
            'Accept: */*',
            'X-API-VERSION: v1',
            'Authorization: Bearer ' . $access_token
        ),
        CURLOPT_POSTFIELDS => json_encode($post_data_sms)
    ));
    $response_sms = curl_exec($ch_sms);

    // Handle the SMS sending response
    if ($response_sms === FALSE) {
        die(curl_error($ch_sms));
    }

    // Close the cURL handle
    curl_close($ch_sms);

    // Handle the response as needed
    $sms_response_data = json_decode($response_sms, TRUE);
    if ($sms_response_data['status'] == 'success') {
        echo "SMS sent successfully!";
    } else {
        echo "SMS sending failed!";
    }

    // Redirect to another page or display a success message
    header("Location: ./biodata.php");
} else {
    // Database entry unsuccessful
    echo "Unsuccessful!!";
}

function login()
{
    global $config_file;
    $post_data = array("username" => "[email protected]", "password" => "XXXXXXXX");
    $ch = curl_init('https://bsms.hutch.lk/api/login');
    curl_setopt_array($ch, array(
        CURLOPT_POST => TRUE,
        CURLOPT_RETURNTRANSFER => TRUE,
        CURLOPT_HTTPHEADER => array(
            'Content-Type: application/json',
            'Accept: */*',
            'X-API-VERSION: v1'
        ),
        CURLOPT_POSTFIELDS => json_encode($post_data)
    ));
    $response = curl_exec($ch);

    // Handle the login response
    if ($response === FALSE) {
        die(curl_error($ch));
    }

    // Close the cURL handle
    curl_close($ch);

    // Decode the JSON response
    $responseData = json_decode($response, TRUE);

    // Check and update the $config_file variable
    if (empty($config_file)) {
        // Update $config_file with a valid file path
        $config_file = 'path/to/config/file.json';
    }

    // Save the login response to the specified file
    file_put_contents($config_file, json_encode($responseData, JSON_PRETTY_PRINT));

    return $responseData['access_token']; // Return the access token
}
?>