phpformmailer is described as 'Easy to use - Nothing stupid your visitors have to type before the form submits'. There are six alternatives to phpformmailer for Online / Web-based, Self-Hosted solutions, Linux, SaaS and PHP. The best alternative is reCAPTCHA, which is free. Other great apps like phpformmailer are hCaptcha (Freemium), PHPMailer (Free, Open Source), Sblam! (Free, Open Source) and Arkose Labs (Paid).
The Arkose Labs platform undermines the economic drivers behind fraud until perpetrators are compelled to abandon attacks. Real-time risk classification of traffic with interactive enforcement challenges, the solution defeats automated attacks.
A negative captcha has the exact same purpose as your run-of-the-mill image captcha: To keep bots from submitting forms. Image ("positive") captchas do this by implementing a step which only humans can do, but bots cannot: read jumbled characters from an image.