PHP Captcha

<?php

$capt = substr(md5(time()),0,4);
$capt .= " " . substr(md5(time()),4,4);

$im = imagecreatetruecolor(125, 45);
$colour = imagecolorallocate($im, 255, 255, 255);

imagefill($im, 0, 0, $colour);

$text_colour1 = imagecolorallocate($im, 80, 80, 80);
$text_colour = imagecolorallocate($im, 160, 160, 160);
$line_colour = imagecolorallocate($im, rand(0, 100), rand(0, 100), rand(150, 255));
$line_colour1 = imagecolorallocate($im, rand(0, 100), rand(0, 100), rand(150, 255));

imagestring($im, 3, 30, 16,  $capt, $text_colour);
imagestring($im, 20, 23, 15,  $capt, $text_colour1);

imageline($im, rand(0, 45), rand(20, 45), rand(0, 145), rand(0, 45), $line_colour);
imageline($im, rand(0, 45), rand(20, 45), rand(0, 145), rand(0, 45), $line_colour1);

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

imagejpeg($im, NULL, 100);

imagedestroy($im);
?>