Simple captcha

gistfile1.php

<?php
session_start();

if (empty($_SESSION['captcha']))
{
	$captcha = mt_rand(10000, 99999);
	$_SESSION['captcha'] = $captcha;
}
else
{
	$captcha = $_SESSION['captcha'];
}

unset ($_SESSION['captcha']);

header ('Content-Type: image/png');

$im = @imagecreatetruecolor(100, 40) or die('GD library not found :(');
$text_color = imagecolorallocate($im, 233, 14, 91);
imagestring($im, 10, 27, 10,  $captcha, $text_color);
imagepng($im);
imagedestroy($im);