One of the best, if not the best, ways to prevent automated web robot tools and spammers from taking advantage of your forms and scripts is by using a captcha image. Here is a simple example that I hacked up earlier today to use on my Advanced Request and Response HTTP Header Viewer among other tools.

View a working example on my Advanced Request and Response HTTP Header Viewer tool.

PHP Anti-Spam Captcha Example

Save this on your site as capya.php and include in your php script with require_once 'capya.php', then run with askapache_captcha(); which outputs XHTML of the captcha image.

';
	} else echo '';


    // destroy image
    imagedestroy($image);

    // delete all captcha images at 12 and 3 oclock if more than 100 are found
    $dt=date('g');
    if(($dt==12)||($dt=='12'))capya_cleanup();
    else if(($dt==3)||($dt=='3'))capya_cleanup();
}

function capya_cleanup(){
$files=glob(CAPYADIR."apache*.jpg");
	if(sizeof($files)>100){
		foreach ($files as $filename) {
    		unlink($filename);
			//echo "$filename size " . filesize($filename) . "n";
		}
	}
}

function capya_string($len){
	$str='';
    for($i=1; $i<=$len; $i++) {
        $ord=rand(48, 90);
        if((($ord >= 48) && ($ord <= 57)) || (($ord >= 65) && ($ord<= 90))) $str.=chr($ord);
        else $str.=capya_string(1);
    }
    return $str;
}
?>

Verify Captcha

To verify a user-submitted (via POST or GET) value for the captcha image, do this.

Get the Anti-Spam Captcha Code

1. askapache-captcha.php
2. Download the TrueType Font files and captcha background image