Build a simple image captcha - PHP Snippet

Build a very basic version of an image CAPTCHA using PHP.

header("Content-type: image/png"); 
$string = "abcdefghijklmnopqrstuvwxyz0123456789"; 
for($i=0;$i<6;$i++){ 
    $pos = rand(0,36); 
    $str .= $string{$pos}; 
} 

$img_handle = ImageCreate (60, 20) or die ("Cannot Create image"); 
//Image size (x,y) 
$back_color = ImageColorAllocate($img_handle, 255, 255, 255); 
//Background color RBG 
$txt_color = ImageColorAllocate($img_handle, 0, 0, 0); 
//Text Color RBG 
ImageString($img_handle, 31, 5, 0, $str, $txt_color); 
Imagepng($img_handle); 

session_start(); 
$_SESSION['img_number'] = $str;