Adding simple math captcha - php

Question

I made captcha by this [tutorial][1],
[1]: http://codechirps.com/how-to-add-a-completely-custom-captcha-to-any-web-form/ but it seems to me it doesn't complete.
I made code but i can send email even if i put wrong answer. I feel that i have to write extra code in php file but i don't know where. Any help greatly appriciated
<div class="modal-body">
<form class="contact" name="contact">
<label class="label" for="name">Имя</label><br>
<input type="text" name="name" class="input-xlarge"><br>
<label class="label" for="email">E-mail</label><br>
<input type="email" name="email" class="input-xlarge"><br>
<label class="label" for="message">Сообщение</label><br>
<textarea name="message" class="input-xlarge"></textarea>
</form>
</div>
<div class="modal-footer">
<p>2 + 3 =</p>
<input type="text" name="captcha" />
<input class="btn btn-warning" type="submit" value="Отправить" id="submit">
Закрыть
<?php
$myemail = '';
if (isset($_POST['name'])) {
$name = strip_tags($_POST['name']);
$email = strip_tags($_POST['email']);
$message = strip_tags($_POST['message']);
$captcha = check_input($_POST['captcha']);
echo "<span class=\"alert alert-success\" >Сообщение отправлено</span><br><br>";
if (!preg_match("/5/", $captcha))
{
show_error("Check your math, Dude");
}
$to = $myemail;
$email_subject = "Contact form submission: $name";
$email_body = "You have received a new message. ".
" Here are the details:\n Name: $name \n ".
"Email: $email\n Message \n $message";
$headers = "From: $myemail\n";
$headers .= "Reply-To: $email";
mail($to,$email_subject,$email_body,$headers);
}?>

Answer 1

Okay, so you need to check the values of your inputs to see if they are valid. If not, you display an error and the mail doesn't get sent. If all the checks pass, the maildoes get sent. So you need to check the $_POST['email'] and the $_POST['captcha'] field (and if you want to, check if the rest isn't empty or whatever).
In php, you can do this like this:
$myemail = "";
if(isset($_POST['name'])){ // check if a form is submitted
if(empty(trim($_POST['name'])) || empty(trim($_POST['message'])) || empty(trim($_POST['email'])) || empty(trim($_POST['captcha'])) ){ // check if values are not empty
echo "Please fill in all the required fields.";
}else if(!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)){ // check email
echo "Please give a real e-mail address.";
}else if(!preg_match("/5/", $_POST['captcha'])){ // the code provided by your script
echo "Get your math right, dude!";
}else{ // all fields seem to be ok
// sanitize input using htmlspecialchars(), see http://stackoverflow.com/a/5788361/1319187
$name = htmlspecialchars($_POST['email']);
$email = $_POST['email']; // email doesn't need to be sanitized since it's been filtered
$message = htmlspecialchars($_POST['message']);
//Send the mail
$to = $myemail;
$email_subject = "Contact form submission: $name";
$email_body = "You have received a new message. ".
" Here are the details:\n Name: $name \n ".
"Email: $email\n Message \n $message";
$headers = "From: $myemail\n";
$headers .= "Reply-To: $email";
if(mail($to,$email_subject,$email_body,$headers)){
echo "Succes! Your mail has been sent!";
}else{
echo "Something went wrong with the sending of a mail.";
}
}
}
Should be rather straightforward, you can google some functions if you don't know what they do.
I also have no idea where check_input() comes from. It's not a native PHP function, and the link you provided doesn't show what it does. Also, the regex to check whether the value of the captcha is 5 is a bit stupid, you could just check $_POST['captcha'] == '5'. Also keep in mind you have to randomize these values a bit.