Simple Captcha in PHP with rand() - php

Question

I'm trying to make a simple captcha in PHP, but it does not work. The query is not currently executing. This is my current code:
<?php
$Random = rand(1, 100);
$Random2 = rand(1,100);
echo "Result: ".$Random." + ".$Random2." ?";
?>
<input type="text" name="r_input"/><br />
$Cap = mysql_real_escape_string($_POST['r_input']);
$Result = $Random+$Random2;
if(isset($_POST['myButton']) and trim($Var) and trim($Var2) and trim($Var3) and $Cap==$Result){
//My Query
}

Answer 1

When you use rand() to generate 2 values, and show those 2 values, and give the form for the user to enter the answer, ...
... the user enters the answer and submits back to the server ...
... the server gets the answer, and then GENERATES 2 NEW VALUES, that don't correspond to the answer given by the user.
Try using session variables to store the generated values in, and match against when the user submits the form!
<?php
session_start();
$captcha_id = 'captcha_' . rand();
$_SESSION['$captcha_id']['val1'] = rand(1,1000);
$_SESSION['$captcha_id']['val2'] = rand(1,1000);
echo "
<form action='' method='post'>
<p>Result: {$_SESSION['$captcha_id']['val1']} + {$_SESSION['$captcha_id']['val2']} = </p>
<input type='hidden' name='captcha_id' value='{$captcha_id}' />
<input type='text' name='captcha_answer' />
<p>?</p>
</form>
";
if (
isset($_POST['captcha_id'])
&& isset($_SESSION[$_POST['captcha_id']])
&& isset($_POST['captcha_answer'])
&& $_SESSION[$_POST['captcha_id']]['val1'] + $_SESSION[$_POST['captcha_id']]['val2'] == intval($_POST['captcha_answer'])
) {
unset($_SESSION[$_POST['captcha_id']]); // don't let this answer be reused anymore.
// do allowed stuff
}
?>

Answer 2

Because $Random and $Random2 have a different value each time.
When you show the form for the first time, they may have the values $Random = 12 and $Random2 = 26. The User sees those, adds them up correctly and types in 38 (which is the correct answer for those two values). The answer is sent to the script again, the values of $Random and $Random2 are generated again (this time as $Random = 23 and $Random2 = 30 which equals 53) and the answer the user has sent is not correct any more.
So you would need to store those values in hidden fields and add these up, instead of the generated ones, like so:
<input type="hidden" name="rand_1" value="<?php echo $Random; ?>">
<input type="hidden" name="rand_2" value="<?php echo $Random2; ?>">
<?php
if ($_POST['rand_1'] + $_POST['rand_2'] == $_POST['r_input']) {
// Query etc.
EDIT: As suggested by #nl-x you should use the Session variables instead of hidden fields to prevent abuse of the captcha:
<?php
$Random = $_SESSION['rand_1'] = rand(1, 100);
$Random2 = $_SESSION['rand_2'] = rand(1,100);
echo "Result: ".$Random." + ".$Random2." ?";
?>
And check those values against the given result afterwards:
<?php
$Cap = mysql_real_escape_string($_POST['r_input']);
$Result = $_SESSION['rand_1'] + $_SESSION['rand_2'];
if ($Result == $Cap) {
// ...

Answer 3

You never re-enter PHP mode after you output your form field:
<input type="text" name="r_input"/><br />
<?php // <----this is missing
$Cap = mysql_real_escape_string($_POST['r_input']);

Answer 4

Pardon me, but you are not making a real captcha. The purpose of the captcha is to distinguish the human from the bots. I would highly suggest you to pick a image database, and randomize a function to call a image. Internally, i would check if the text/description of the image matches with what the user typed.
The only thing you will rand() is what image to load from your image database.
That's a not-healthy way to do it, and there are plenty of better ways to do this. But it's more closer to a captcha than just your current code.
There is also a lot of libraries and engines that can do the job for you.
I'm not a pro at PHP, or even programming at all, but i think you're going to the wrong side - your code won't block any... malicious actions at all, or whatever kind of action that you will try to prevent with the captcha.
Search google for the libraries. PhpCaptcha is one of them. And here is a very simple quickstart guide for phpcaptcha.
Here's a code example, extracted from PHPCaptch that I linked above.
At the desired position in your form, add the following code to display the CAPTCHA image:
<img id="captcha" src="/securimage/securimage_show.php" alt="CAPTCHA Image" />
Next, add the following HTML code to create a text input box:
<input type="text" name="captcha_code" size="10" maxlength="6" />
[ Different Image ]
On the very first line of the form processor, add the following code:
<?php session_start(); ?>
The following php code should be integrated into the script that processes your form and should be placed where error checking is done. It is recommended to place it after any error checking and only attempt to validate the captha code if no other form errors occured. It should also be within tags.
include_once $_SERVER['DOCUMENT_ROOT'] . '/securimage/securimage.php';
$securimage = new Securimage();
This includes the file that contains the Securimage source code and creates a new Securimage object that is responsible for creating, managing and validating captcha codes.
Next we will check to see if the code typed by the user was entered correctly.
if ($securimage->check($_POST['captcha_code']) == false) {
// the code was incorrect
// you should handle the error so that the form processor doesn't continue
// or you can use the following code if there is no validation or you do not know how
echo "The security code entered was incorrect.<br /><br />";
echo "Please go <a href='javascript:history.go(-1)'>back</a> and try again.";
exit;
}
Following the directions above should get Securimage working with minimal effort.
This code is included here as well.
Good luck!