CAPTCHA stands for ” Completely Automated Public Turing test to tell Computers and Humans Apart”.

It’s required that out website, forms should be SPAM free, and automated bots should not be able to fill forms and send data to server to increase load of our server.

So today I am writing a Simple Arithmetic CAPTCHA creation tutorial.This tutorial is not FULL FLEDGED, i.e. it’s implementation is dependent upon you. How you want to , where you want to that totally depends on you.

We will be using PHP Image Library GD2 Library. You need to have it enabled, which you can enable from PHP.INI file.

The code for Simple Arithmetic CAPTCHA File goes below:

Source code

<?php
session_start();
header("Content-type: image/png");
$x=rand(1,10); $y=rand(1,10);
        $z=rand(1,2);
 
        if($z==1)
           {
            $_SESSION['captcha']="+";
             $res=$x+$y;
           }
           else
            {
            $_SESSION['captcha']="-";
             if($x>$y)
             {
                 $res=$x-$y;
             }
             else
             {
                 $res=$y-$x;
             }
            }
        if($x > $y)
        {
          $str= "Enter $x".$_SESSION['captcha']."$y";
 
        }
        else
        {
        $str="Enter $y".$_SESSION['captcha']."$x";
        }
 
$img_handle = ImageCreate (100, 20) or die ("Cannot Create image");
//Image size (x,y)
$back_color = ImageColorAllocate($img_handle, 255, 255, 255);
//Background color RBG
$txt_color = ImageColorAllocate($img_handle, 0, 0, 0);
//Text Color RBG
ImageString($img_handle, 31, 5, 0, $str, $txt_color);
Imagepng($img_handle);
unset($_SESSION['img_number']);
$_SESSION['img_number'] = $res;        
 
?>

I am saving this file with name captcha.php.

To access this CAPTCHA generated , you have to save the file inside

c:\wamp\www\

folder, I am saving it at

c:\wamp\www\cap

To check whether the file is working or not, type

http://localhost/cap/captcha.php

The above path is according to my settings, you may vary according to location where you save.

You will get output like

simple arithmetic captcha output

Now let’s write a form where we will actually use this simple arithmetic captcha written in PHP.

form doesn’t consist of huge number of fields, in this example , I am just asking user to enter CAPTCHA itself in the input box.

CODE for HTML file:

Source code

<form name='f1' action='validate.php' method='post'>
<fieldset>
<img src='captcha.php'><input type='text' name='cap'>
</fieldset>
<input type='submit' name='submit' value='submit'>

I am saving it as form.html inside

c:\wamp\www\cap

Now let’s create validate.php to validate ARITHMETIC CAPTCHA ‘s Result.

Source code

<?php
  session_start();
  if(isset($_POST['cap']))
  {
      if($_SESSION['img_number']== $_POST['cap'])
      {
          echo "You entered correct ARITHMETIC CAPTCHA";
      }
      else
      {
          echo "You entered wrong ARITHMETIC CAPTCHA, Retry.";
      }
  }
?>

I have saved this file validate.php inside

c:\wamp\www\cap

Now let’s have a look at output:

FIRST WRONG INPUT

captcha form with wrong input

captcha form with wrong output error

RIGHT INPUT

captcha form with right input

captcha form with right output message

How does this Simple Arithmetic CAPTCHA thing work??

Let’s have a look at codes.

<?php
session_start();
header("Content-type: image/png");
$x=rand(1,10); $y=rand(1,10);
        $z=rand(1,2);

        if($z==1)
           {
            $_SESSION['captcha']="+";
             $res=$x+$y;
           }
           else
            {
            $_SESSION['captcha']="-";
             if($x>$y)
             {
                 $res=$x-$y;
             }
             else
             {
                 $res=$y-$x;
             }
            }
        if($x > $y)
        {
          $str= "Enter $x".$_SESSION['captcha']."$y";

        }
        else
        {
        $str="Enter $y".$_SESSION['captcha']."$x";
        }

$img_handle = ImageCreate (100, 20) or die ("Cannot Create image");
//Image size (x,y)
$back_color = ImageColorAllocate($img_handle, 255, 255, 255);
//Background color RBG
$txt_color = ImageColorAllocate($img_handle, 0, 0, 0);
//Text Color RBG
ImageString($img_handle, 31, 5, 0, $str, $txt_color);
Imagepng($img_handle);
unset($_SESSION['img_number']);
$_SESSION['img_number'] = $res;        

?>

This was code for CAPTCHA.PHP file

what happens in it?

session_start() -> this method starts new session.

header(“content-type:image/png”); -> Tells browser that Output is not HTML , rather it’s an IMAGE.

we have used rand(1,10) function-> rand() takes two int arguments, and generates pseudo random number in given range, here given range is 1 to 10.

then in if() conditions, I have checked, out of two numbers, $x and $y , which is greatest, and put it first , it will allow easy non negative subtraction.

$_SESSION[‘captcha’] is saving what operation is performed, is it addition or subtraction, which is then used to append to message.

$img_handle is a variable, which is used to hold a raw image, of width and height specified as 100,20 respectively. If it fails while creating , we are stopping script execution.

ImageColorAllocate($img_handle, 255, 255, 255); -> Is used to give white color to image, we are storing it in $back_color variable.

ImageColorAllocate($img_handle, 0, 0, 0); -> Is used to give text color as black.  stored in $txt_color variable.
ImageString($img_handle, 31, 5, 0, $str, $txt_color);-> This function writes the question on image, i.e. string is written on image.
Imagepng($img_handle);-> Gives image in PNG format.

$_SESSION[‘img_number’]=$res -> It has saved what is written there on CAPTCHA. i.e. the result of the numbers and operation.

HTML form code is straight forward, it just consists of

<img src=’captcha.php’> , as above file generates Image output, we are directly embedding it in <img> tag.

When user is submitting , we are going on validate.php page;

Source code

<?php
  session_start();
  if(isset($_POST['cap']))
  {
      if($_SESSION['img_number']== $_POST['cap'])
      {
          echo "You entered correct ARITHMETIC CAPTCHA";
      }
      else
      {
          echo "You entered wrong ARITHMETIC CAPTCHA, Retry.";
      }
  }
?>

What are we doing here? Simple, we have saved result in session super global.  so $_POST[‘cap’] consists of input given by user, which we are comparing. if results match, it’s correct, else error is displayed as output.

Note: We have just selected + and – operations here, you can use ‘x’ and ‘/’ operations, but for sake of simplicity , + and – operations are preferred, along with , it just takes number between 1 to 10 so that it will be easy to perform operation.

Hope you liked SIMPLE ARITHMETIC CAPTCHA in PHP , and hopefully you will find it useful.