<div class="formlabel">Demuestre que Vd. no es un espamer, escriba en letras el resultado de sumar tres y siete (en minúsculas):</div>
<div class="formfield"><input type="text" name="Filtro" size="4" /></div>
<input type=”hidden” name=”Solucionfiltro” value=”diez” >
However, the hidden field still shows on the browser (it doesn’t show the value but it shows an empty square) and I can’t get the form check to compare the values of “Filtro” and “Solucionfiltro”.
You could indeed use Captcha as Paul suggested, another option would be using server side scripting, in your case PHP which you already use for your pages anyway
I wouldn’t recommend using javascript for your captcha because it won’t work in browsers with javascript turned off. Google took over reCaptcha some time ago and it’s one of the better free, easy to implement captchas. The only downside of reCaptcha is that it isn’t very flexible regarding styling, but depending on your skill level in php, you might consider building your own customised captcha.