Image creation in php

session_start();
header("Content-type: image/png");

$pattern = "123456789abcdefghijkmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
for($i=0;$i<7;$i++)
{
$rndnum.= $pattern{rand(0,33)};
}
//Registering Sessions Starts Here
$_SESSION['SecureCode']=$rndnum;
session_register('SecureCode');
//Ends Here


$im = @imagecreate(140, 30)
or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 47, 79, 79);
$text_color = imagecolorallocate($im, 255, 255, 255);
$font = imageloadfont('assets/images/chowfun.gdf');
imagestring($im, $font, 15, 3, "$rndnum", $text_color);
imagepng($im);
?>