Simple php captcha

/* CAPTCHA FILE */
session_start();

$_SESSION["rand"] = substr(str_shuffle("ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyz"), 0, 6);

$im = imagecreatetruecolor(62, 25);
$bg = imagecolorallocate($im, 0, 140, 186);
$fg = imagecolorallocate($im, 124, 119, 117);

imagefill($im, 0, 0, $bg);
imagestring($im, 5, 5, 5,  $_SESSION["rand"], $fg);

header("Cache-Control: no-cache, must-revalidate");
header('Content-type: image/png');

imagepng($im);
imagedestroy($im);

/* CAPTCHA FILE END */

/* FORM FILE */

session_start();

// some code ...

if(isset($_POST["captcha"]) && !empty($_POST["captcha"]) && !is_array($_POST["captcha"]))
    return $_POST["captcha"] == $_SESSION["rand"] ? true : false;