Validate a recaptcha token with wordpress plugin contact form 7, useful when integrating with a custom frontend.

cf7-recaptcha-v3.php

<?php

/* Verificacion especial recaptcha - cf7 */

add_filter('wpcf7_spam', function ($spam) {

  if ($spam) {
    return $spam;
  }

  $submission = WPCF7_Submission::get_instance();

  if ("" == trim($_POST['g-recaptcha-response'])) {
    $spam = true;
    $submission->add_spam_log([
      'agent' => 'custom_recaptcha_v3',
      'reason' => "Empty 'g-recaptcha-response'",
    ]);
  } else {
    $token = $_POST['g-recaptcha-response'];
    $secret = 'YOUR_RECAPTCHA_SECRET_KEY';
    // verify recaptcha response
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,"https://www.google.com/recaptcha/api/siteverify");
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(['secret' => $secret, 'response' => $token]));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($ch);
    curl_close($ch);
    $responseArray = json_decode($response, true);
    // validate response
    if ($responseArray["success"] != true) {
      $spam = true;
      $error = empty($responseArray["error-codes"]) ? "Undefined error" : join(", ", $responseArray["error-codes"]);
      $submission->add_spam_log([
        'agent' => 'custom_recaptcha_v3',
        'reason' => "recaptcha error: $error",
      ]);
    }
  }
  return $spam;
}, 10, 1);