simple captcha in a php email form

[thesimplecreator]

on my free web hosting site people sign up with a PHP email form.
ive gotten a few spam emails lately and was wondering, in php is there a way i could set up a captcha, or something they have to enter before the form lets them send it.

the script i currently use is:

php:

<?php
if(isset($_POST['submit'])) {

$to = "deathnerdhosting+test@gmail.com";
$subject = "signup";
$email_field = $_POST['email'];
$name_field = $_POST['name'];
$pass_field = $_POST['pass'];
$message = $_POST['message'];
 
$body = "From: $email_field\n E-Mail: $email_field\n NAME: $name_field\n PASS: $pass_field\n MESSAGE:\n $message";
 
echo "Your Email has been submitted. Return to http://www.deathnerd.com/index.html";


mail($to, $subject, $body);

} else {

echo "You cannot access mailer.php directly you must go to signup.html";

}
?>

and the html to the form is:

<table width=200 height=400 border=1>
<tr>
<td>
<h3>SIGNUP</h3>
<form method="POST" action="mailer.php">
   <tt>Email Address</tt>
   <br>
   <input type="text" name="email" size="19"><br>
   <br>
   <tt>Desired Account/Website Name</tt>
   <br>
   <input type="text" name="name" size="19"><br>
   <br>
   <tt>Desired Password</tt>
   <br>
   <input type="text" name="pass" size="19"><br>
   <br>
   <tt>Questions or a request to have me set up phpbb3 forums on your site.</tt>
   <br>
   <textarea rows="7" name="message" cols="25"></textarea>
   <br>
   <br>
   <input type="submit" value="Submit" name="submit">
</form>

any help would be greatly appreciated, thanx