I need help understanding if some PHP code does what I think it does.

We are replacing our old Contact Form (which had no safety measures) and want it to have two safety features:
1) a simple security code question, like "Using only numbers, what is 10 plus 15?"
2) a way of cleaning, or sanitizing the submitted data to remove potentially damaging hacker codes

We think we found both - but we are not sure

I found a pre-made contact form here: http://www.freecontactform.com/free.php that definitely has feature #1, but I can't tell if it has feature #2

Below are their two pieces of pre-made PHP code that I think are applicable.

We need to know if:
a) it will work with PHP version 5+
b) the #2 (anti-spammer/anti-hacker) features* are in there


*for my educational reasons, can you tell me which lines of code are the anti-hacker ones?

Thank you for your help

freecontactformsettings.php
PHP Code:
<?php
$email_to 
"youremailaddress@yourdomain.com"// your email address
$email_subject "Contact Form Message"// email subject line
$thankyou "thankyou.htm"// thank you page
// if you update the question on the form -
// you need to update the questions answer below
$antispam_answer "15";
?>
freecontactformprocess.php
PHP Code:
<?php
/**
 * 
 * URL: [url]www.freecontactform.com[/url]
 * 
 * Version: FreeContactForm Free V2.1
 * 
 * Copyright (c) 2012 Stuart Cochrane
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 * 
 * 
 * Note: This is NOT the same code as the PRO version
 * 
 */

if(isset($_POST['Email_Address'])) {
    
    include 
'freecontactformsettings.php';
    
    function 
died($error) {
        echo 
"Sorry, but there were error(s) found with the form you submitted. ";
        echo 
"These errors appear below.<br /><br />";
        echo 
$error."<br /><br />";
        echo 
"Please go back and fix these errors.<br /><br />";
        die();
    }
    
    if(!isset(
$_POST['Full_Name']) ||
        !isset(
$_POST['Email_Address']) ||
        !isset(
$_POST['Telephone_Number']) ||
        !isset(
$_POST['Your_Message']) || 
        !isset(
$_POST['AntiSpam'])        
        ) {
        
died('Sorry, there appears to be a problem with your form submission.');        
    }
    
    
$full_name $_POST['Full_Name']; // required
    
$email_from $_POST['Email_Address']; // required
    
$telephone $_POST['Telephone_Number']; // not required
    
$comments $_POST['Your_Message']; // required
    
$antispam $_POST['AntiSpam']; // required
    
    
$error_message "";
    
    
$email_exp '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
  if(
preg_match($email_exp,$email_from)==0) {
      
$error_message .= 'The Email Address you entered does not appear to be valid.<br />';
  }
  if(
strlen($full_name) < 2) {
      
$error_message .= 'Your Name does not appear to be valid.<br />';
  }
  if(
strlen($comments) < 2) {
      
$error_message .= 'The Comments you entered do not appear to be valid.<br />';
  }
  
  if(
$antispam <> $antispam_answer) {
    
$error_message .= 'The Anti-Spam answer you entered is not correct.<br />';
  }
  
  if(
strlen($error_message) > 0) {
      
died($error_message);
  }
    
$email_message "Form details below.\r\n";
    
    function 
clean_string($string) {
      
$bad = array("content-type","bcc:","to:","cc:");
      return 
str_replace($bad,"",$string);
    }
    
    
$email_message .= "Full Name: ".clean_string($full_name)."\r\n";
    
$email_message .= "Email: ".clean_string($email_from)."\r\n";
    
$email_message .= "Telephone: ".clean_string($telephone)."\r\n";
    
$email_message .= "Message: ".clean_string($comments)."\r\n";
    
$headers 'From: '.$email_from."\r\n".
'Reply-To: '.$email_from."\r\n" .
'X-Mailer: PHP/' phpversion();
mail($email_to$email_subject$email_message$headers);
header("Location: $thankyou");
?>
<script>location.replace('<?php echo $thankyou;?>')</script>
<?php
}
die();
?>