Channels ▼

Web Development

Implementing Audio CAPTCHA

Source Code Accompanies This Article. Download It Now.

They're Playing Our Song

The PlaySound.php script (Listing Two) uses PHP to dynamically form an MP3 stream by concatenating shorter MP3 files. The PHP header() function writes out the content-type as MPEG. Then, I start the stream with a short introduction that simply says, "Please enter these four numbers." This is the Intro.mp3 file, which starts the stream.

The script then retrieves the four numbers from the $_SESSION array. The numbers are stored in the array as a single string, so the script pulls the string apart, storing the numbers in a filename array. Then, for each number, the corresponding MP3 file is added to the stream. This is the section where you may want to include some additional words.

Finally, the stream is printed to standard out and, thanks to the content-type header, is interpreted by the browser to be an MP3 file. Naturally, there are many different ways to play audio on the Web. I chose MP3 because it's easy to concatenate separate files due to the fact that there is no header information to be concerned with as there is with .wav files. You may want to experiment with different file types for your own CAPTCHA, perhaps Flash for example.

header('Content-type: audio/x-mpeg');
/* put an intro in first */
$thisFile = "Intro.mp3";
$fh = fopen($thisFile, 'r');
$allAudio = fread($fh, filesize($thisFile));

/* add the numbers to the file */
$str = $_SESSION['captchaAnswer'];
/* this works under PHP4,
can use str_split() here instead in PHP5 */
$fileName = array();
for ($i = 0; $i < strlen($str); $i++) 
    $fileName[] = substr($str,$i,1);
foreach ($fileName as $fileNumer)
    $thisFile = $fileNumer . ".mp3";
    $fh = fopen($thisFile, 'r');
    $allAudio .= fread($fh, filesize($thisFile));
/* output the resulting MP3 */
print $allAudio;
Listing Two

The last file, CaptchaSubmit.php (Listing Three) serves to check user input and outputs a simple message to indicate success or failure. CheckAnswer() first checks to see if anything was posted at all and if the PHP session is set. Then, it checks the user posted answer against the challenge initialized in the index.htm file, returning True or False (1 or 0) as is appropriate.



header('Content-type: text/html');

// true if correct answer
function CheckAnswer()
  if (!isset($_POST['userAnswer']) || !isset($_SESSION['captchaAnswer']))
    return 0;
  else if ($_POST['userAnswer'] == $_SESSION['captchaAnswer']) 
    return 1;
  return 0;  
print "<html><head></head><body>";
print $_POST['userAnswer'];
    print " is correct.";
    print " is not correct, please go back and try again.";
Listing Three

You will want to replace the lower half of this script, that outputs success or failure, with your own success criteria, probably a redirect back to your blog or other media page.

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.