PHP制作验证码

session_start();
$a = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t',
'u','v','w','x','y','z','1','2','3','4','5','6','7','8','9','0',);
$r = "";
for($i = 1;$i<=4;$i++){
$r .= $a[rand(0,count($a)-1)];
}
$_SESSION['r'] = $r;
$i = imagecreatetruecolor(100,35);
$bg = imagecolorallocate($i,0,0,0);
$font = imagecolorallocate($i,255,255,255);
imagestring($i,5,35,12,$r,$font);
ob_clean();
header("Content-type:image/gif");

imagegif($i);
?>