Do you have a contact form on your site and you get spammed by bots constantly?
With this code you create contact form with a captcha image, and that will prevent bots

######################
##### contact.php ####
######################
<?php
@session_start();
if(isset($_POST['submit'])) {

if(($_POST['security_code'] == $_SESSION['security_code']) && (!empty($_SESSION['security_code'])) ) {

$to = "YOUR_MAIL_HERE";
$subject = "Form submission";
$name_field = $_POST['name'];
$email_field = $_POST['email'];
$message = $_POST['message'];

$body = "From: $name_field\n E-Mail: $email_field\n Message:\n $message";

mail($to, $subject, $body);
echo "Email was great success! I will email you back soon.";
unset($_SESSION['security_code']);
}
else {
unset($_SESSION['security_code']);
echo "Incorrect captcha code.";
}
}
?>
<p>
<form method="POST" action="">
<p>Your Name:<br />

<input type="text" name="name" size="19">
<br>
Your Email:<br />

<input type="text" name="email" size="19">
<br>
Your Message:<br />

<textarea rows="9" name="message" cols="30"></textarea>
</p>
<p><img src="captcha.php" /> <br>

Type the letters from the image above into this box:<br />
(This prevents Bots from sending me spam.)<br />
<input id="security_code" name="security_code" type="text" />
<br>
<input type="submit" value="Submit" name="submit">
</p>
</form>

#######################
##### captcha.php #####
#######################

<?php
session_start();

class CaptchaSecurityImages {

var $font = 'monofont.ttf';

function generateCode($characters) {
/* list all possible characters, similar looking characters and vowels have been removed */
$possible = '23456789bcdfghjkmnpqrstvwxyz';
$code = '';
$i = 0;
while ($i < $characters) {
$code .= substr($possible, mt_rand(0, strlen($possible)-1), 1);
$i++;
}
return $code;
}

function CaptchaSecurityImages($width='120',$height='40',$characters='6') {
$code = $this->generateCode($characters);
/* font size will be 75% of the image height */
$font_size = $height * 0.75;
$image = imagecreate($width, $height) or die('Cannot initialize new GD image stream');
/* set the colours */
$background_color = imagecolorallocate($image, 255, 255, 255);
$text_color = imagecolorallocate($image, 20, 40, 100);
$noise_color = imagecolorallocate($image, 100, 120, 180);
/* generate random dots in background */
for( $i=0; $i<($width*$height)/3; $i++ ) {
imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
}
/* generate random lines in background */
for( $i=0; $i<($width*$height)/150; $i++ ) {
imageline($image, mt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color);
}
/* create textbox and add text */
$textbox = imagettfbbox($font_size, 0, $this->font, $code) or die('Error in imagettfbbox function');
$x = ($width - $textbox[4])/2;
$y = ($height - $textbox[5])/2;
imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code) or die('Error in imagettftext function');
/* output captcha image to browser */
header('Content-Type: image/jpeg');
imagejpeg($image);
imagedestroy($image);
$_SESSION['security_code'] = $code;
}

}

$width = isset($_GET['width']) && $_GET['height'] < 600 ? $_GET['width'] : '120';
$height = isset($_GET['height']) && $_GET['height'] < 200 ? $_GET['height'] : '40';
$characters = isset($_GET['characters']) && $_GET['characters'] > 2 ? $_GET['characters'] : '6';

$captcha = new CaptchaSecurityImages($width,$height,$characters);

?>