Secure Number Image Creator: Generate an image with a random number or text

Secure Number Image Creator:
This is a simple class that can be used to generate an image with a random number or text.

How to use(my example--See the example.php source code):

1) use img.php as a picture:
<img src="img.php" />

2) (in example.php)check $_SESSION['key'] and compare with $_POST['number']

if($_POST['number']==$_SESSION['key']){
//Do
}