Random number generator

How can I make a random number generator pop up everytime someone visits my site?

 

Every time, same person, how many digits ?

Explain more....

EDIT:

Without further details, I don't know if this helps ?

<?php
function genRandomString($length) {
    $characters = "0123456789";  
    for ($p = 0; $p < $length; $p++) {
        $string .= $characters[mt_rand(0, strlen($characters))];
    }
    return $string;
}

$string = genRandomString(5); // Not more then 5 digits

echo $string;

?> 

PHP:

Just <?php echo $string; ?> where you want it to appear