Captcha PHP

 <?php

session_start();

$code=rand(1000,9999);

$_SESSION["code"]=$code;

$im = imagecreatetruecolor(50, 24);

$bg = imagecolorallocate($im, 22, 86, 165); //background color blue

$fg = imagecolorallocate($im, 255, 255, 255);//text color white

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

imagestring($im, 5, 5, 5,  $code, $fg);

header("Cache-Control: no-cache, must-revalidate");

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

imagepng($im);

imagedestroy($im);

?>