session_start();
header("Content-type: image/png");
$width = 146;
$height = 30;

$im = @imagecreate($width, $height)or die("Cannot Initialize new GD image stream");

imagecolorallocate($im, 255, 250, 255);
$noise_color = imagecolorallocate($im, 207, 239, 250);
for($i=0; $i<($width*$height)/3; $i++) {
	imagefilledellipse($im, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
}

/* generate random lines in background */
for($i=0; $i<($width*$height)/150; $i++ ) {
 imageline($im, mt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color);
}

$text_color[0] = imagecolorallocate($im, 255, 0, 0);
$text_color[1] = imagecolorallocate($im, 51, 166, 207);

$text = mb_substr(uniqid(mt_rand(), true), 0, 6);
unset($_SESSION['captcha']);
$_SESSION['captcha']['blue'] = $_SESSION['captcha']['red'] = '';

for($j = 0; $j < mb_strlen($text); $j++) {
	imagettftext($im, 20, 0, 5+($j*23), 24, $text_color[$j%2], './offshore.ttf', $text[$j]);
	if(($j%2) == 1) {
		$_SESSION['captcha']['blue'] .= $text[$j];
	}
	else {
		$_SESSION['captcha']['red'] .= $text[$j];
	}
}
imagepng($im);
imagedestroy($im);

http://vladimir-ivanov.net

Tell the users to enter only the red or only the blue digits, not everything they see.
Most bots will try to enter everything, displayed on the images.
To check is captcha code entered correctly, use $_SESSION['captcha']['red'] to check the red digits only and $_SESSION['captcha']['blue'] to check the blue digits only. Good luck !

Captcha with color protection

PHP