<?php
header('Content-type: image/png');
session_start();

function random_string($num)
{
	$string = 'ABCDEFGHIJKLMNPQRSTUVWXYZ123456789';
	$return = '';
	while($num-- > 0)
		$return .= $string[rand(0, strlen($string) - 1)];
	return $return;
}

$text = random_string(8);

$_SESSION['NUM'] = sha1($text);

$image = @imagecreatefrompng('image.png'); 
$text_color = @imagecolorallocate($image, 0, 0, 0);

for($i = 0; $i < strlen($text); $i++)
	@imagestring($image, 5, 35 + ($i*10), round(rand(15,25)), $text[$i], $text_color);

@imagepng($image);
@imagedestroy($image);
?>