Joomla RSForm Honeypot inspired by https://webaim.org/blog/spam_free_accessible_forms/

_joomla-rsform-honeypot

A small honeypot method to catch spam bots.
Inspired by blog post on https://webaim.org/blog/spam_free_accessible_forms/

Form Fields > textbox

Name: usefulname
Caption: usefulName

Form Properties > CSS

<style>
.rsform-block-usefulname{
  display:none;
  visibility:hidden;
}
</style>

Form Properties > Script called on form process

/**
 * https://webaim.org/blog/spam_free_accessible_forms/
 */
$spam = false;

// Detect form elements for the most common header injections and other code
if (preg_match( "/bcc:|cc:|multipart|\[url|Content-Type:/i", implode($_POST['form']))) 
{
    $spam=true;
}

// Detect more than 3 outgoing links
if (preg_match_all("/<a|https?:/i", implode($_POST['form']), $out) > 3) 
{
    $spam=true;
}

// Detect content within a hidden form element
if( !empty($_POST['form']['usefulname']))
{
  $spam = true;
}

// Ensure the form is posted from your server
//if((isset($_SERVER['HTTP_REFERER']) && stristr($_SERVER['HTTP_REFERER'],$_SERVER['HTTP_HOST']))) {
//    $spam=true;
//}

// Sent spammer to somewhere else
if ($spam) 
{
  header('Location: https://www.example.com/');
  die;  
}
// END Honeypot

@kbrookes fixes applied. Thank you