Verifying solutions in PHP

<?php

$curl = curl_init();

$secret_key = "sk_xxxxxxxx";
$solution = "..."; // Extract the solution from the request body (_botpoison)

$data = array("secretKey" => $secret_key, "solution" => $solution);
$data_string = json_encode($data);

curl_setopt_array($curl, [
    CURLOPT_URL => "https://api.botpoison.com/verify",
    CURLOPT_CUSTOMREQUEST => "POST",
    CURLOPT_POSTFIELDS => $data_string,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => array('Content-Type: application/json', 'Content-Length: ' . strlen($data_string))
]);

$response = curl_exec($curl);
$error = curl_error($curl);

curl_close($curl);

if ($error) {
    echo "cURL Error #:" . $error;
} else {
    if (json_decode($response)->ok) {
        echo "Not spam: allow request";
    } else {
        echo "Spam: reject request";
    }
}