[Force Users To Type Specific Word In Php Form]

I have this nicely working form code for my website, but want to add my own simple captcha.

 

I want to add another field that forces users to type out what they see in a static image that I will insert - so there is only one right answer.

 

If, for example, name required the word 'stan' to be added, how do I force users to do this?

 

Many thanks in advance!

 

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

$name = $_POST['name']; // required
$telephone_number = $_POST['telephonenumber']; // required

if($name=='' ||
$telephone_number=='' )

{

die('You have not filled in all of the required fields - please hit your back button and try again!');

}

else{
$email_to = "my@email.com";
 $email_subject = "Contact from company";
$email_message = "Form details below.\n\n";

function clean_string($string) {
 $bad = array("content-type","bcc:","to:","cc:","href");
 return str_replace($bad,"",$string);
}

$email_message .= "Name: ".clean_string($name)."\n";
$email_message .= "Telephone Number: ".clean_string($telephone_number)."\n";

// create email headers
$headers = 'From: Company'.
//'Reply-To: '.$email_from."\r\n" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);
}
}
?>