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Captcha image verification

A good way to avoid automatic form submissions when creating a web form is to add some kind of verification. One of the best ways is to use an image verification, called also captcha. What it does is to dynamically create an image with a random string displayed on it. Then visitor is asked to type that string in a text field and once the form is submitted it checks if the string on the image matches the one inputted by the user. Because there is no easy way to read a text from an image (image recognition) this is a good way to protect your web forms from spammers.
For doing this CAPTCHA I would suggest using a session variable where you store the string generated and displayed on that dynamically generated image.

$text = rand(10000,99999);
$_SESSION["vercode"] = $text;
$height = 25;
$width = 65;

$image_p = imagecreate($width, $height);
$black = imagecolorallocate($image_p, 0, 0, 0);
$white = imagecolorallocate($image_p, 255, 255, 255);
$font_size = 14;

imagestring($image_p, $font_size, 5, 5, $text, $white);
imagejpeg($image_p, null, 80);

Save this code in a file called captcha.php. What this script does is to generate a random number from 10000 to 99999 and then assign it to $_SESSION['vercode']. Then it generates a 25x65 pixels image with black background and white text using size 14. So if you upload that captcha.php file on your web site and open http://www.site.com/captcha.php you will see an image displaying random integer. You will receive a new random integer every time you refresh that page.

Next we need to create our web form.

<form action="submit.php" method="post"> 
Comment: <textarea name="coment"></textarea><br>
Enter Code <img src="captcha.php"><input type="text" name="vercode" /><br>
<input type="submit" name="Submit" value="Submit" />

Above code will create a form with a single textarea box, randomly generated image using the captcha.php script and a text field where you will have to enter the verification code.

All we have to do now is to make the submit.php script which will check if the verification code you enter matches the one that has been randomly generated.

if ($_POST["vercode"] != $_SESSION["vercode"] OR $_SESSION["vercode"]=='') {
echo '<strong>Incorrect verification code.</strong><br>';
} else {
// add form data processing code here
echo '<strong>Verification successful.</strong><br>';

239 Comments to "Captcha image verification"

  • Gupta Pedamallu

    Gupta Pedamallu

    June 25, 2013 at 14:27 pm

    thanku.. so much

  • shail


    June 13, 2013 at 16:23 pm

    i need help on following ....it showing line 4 has a error

    if (!function_exists('bcmodpow'))
    function bcpowmod($x, $y, $mod);{return bcmod(bcpow($x, $y), $mod);};

    could you help

  • Parthasarathy


    April 26, 2013 at 15:50 pm

    Please give codes for captcha.php and submit.php fully with example output.


  • xcer1212


    March 16, 2013 at 20:35 pm

    to make dirty images :)

    $lines = mt_rand(1,5);

    for( $i=0; $i<$lines; $i++ ) {
    mt_rand(0,$width), mt_rand(0,$height),
    mt_rand(0,$width), mt_rand(0,$height),
    imagecolorallocate($image_p, mt_rand(150,255), mt_rand(150,255), mt_rand(150,255)));

  • Adebayo Michael

    Adebayo Michael

    March 4, 2013 at 03:19 am

    please help i got a parse error at line 8, Where we have $image_p = imagecreate($width, $height); I checked the previous line and it was ok. please assist. Thanks

  • kalitso


    February 2, 2013 at 17:02 pm

    i cant understand despite un put correct code i receive this error :

    Incorrect verification code ............ :(

  • Joshua


    December 3, 2012 at 20:30 pm

    can i use this in php mvc codeigniter?

  • Balveer kumar

    Balveer kumar

    November 5, 2012 at 08:16 am


    please provide the string capcha code like "ATcb1234"

    my code is bellow but its an error

    session_start ();
    $t1= rand(A,Z);
    $t2= rand (a,z);
    $t3= rand(100,500);
    $tx= "$t1"."$t2"."$t3" ;
    $_SESSION['cap']= $tx;
    header ("content-type: image/jpeg");
    $w =80;
    $image=imagecreate ($w,$h);
    $b1=imagecolorallocate ($image, 00,00,00);
    $ww=imagecolorallocate ($image, 255,255,255);
    imagestring ($image,$f,7,7,$tx,$ww);
    imagejpeg ($image, null, 50);

    • Makc


      December 29, 2012 at 17:11 pm

      // IMPORTANT!
      // Send ALL headers before session_start() is invoked
      header ("content-type: image/jpeg");

      session_start ();

      // Function returns random symbol string
      // Accepts symbol quantity of the returned string
      function randomCaptcha($symbol_qty){

      // Use range() instead of random for strings
      $symbol_list = array_merge(range('a', 'z'), range('A', 'Z'), range('0', '25'));
      foreach (array_rand($symbol_list, $symbol_qty) as $v) {
      $random_captcha .= $symbol_list[$v];
      return $random_captcha;

      // Get a random string with 5 characters in it
      $tx = randomCaptcha(5);

      // Thats all, here goes your code ;)
      $_SESSION['cap']= $tx;

      $w =80;
      $image=imagecreate ($w,$h);
      $b1=imagecolorallocate ($image, 00, 00, 00);
      $ww=imagecolorallocate ($image, 255, 255, 255);
      imagestring ($image,$f,7,7,$tx,$ww);
      imagejpeg ($image, null, 50);

  • aamir


    October 18, 2012 at 14:18 pm

    thanks beven and pratik its work for me

  • beven


    October 18, 2012 at 14:16 pm

    there will be a two file
    2) captcha.php

    works fine thanks dude.

    remove # from this line in index.php file

    <td id="reloadLink">
    <a href="" >Reload</a>

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