Unique, unpredictable, 12 digit, integer id

Question

How would I go about generating this... I want to keep my primary key sequential and have a 12 digit unique pin generated for each new object added to the database.

Answer 1

<?php 
$allowed_characters = array(1,2,3,4,5,6,7,8,9,0);
for($i = 1;$i <= 12; $i++){
    $pass .= $allowed_characters[rand(0, count($allowed_characters) - 1)];
}
echo $pass;
?>

demo: http://sandbox.phpcode.eu/g/c0190/4