How to create a simple captcha

<?php
session_start();
$code=rand(1000,9999);
$_SESSION["code"]=$code;
$image=imagecreatetruecolor(50,24);
$background_color=imagecolorallocate($image,22,100,165);
$forground_color=imagecolorallocate($image,225,225,225);
imagefill($image,0,0,$background_color);
imagestring($image,5,5,5,$code,$forground_color);
header("Cache-Control:no-cache,must-revalidate");
header('Content-type: image/png');
imagepng($image);
imagedestroy($image);
?>


call this file in <img src="above file.php" />