For Zero fill - just use the sprintf() function

$pr_id = 1;
$pr_id = sprintf("%03d", $pr_id);
echo $pr_id;

//outputs 001
-----------------

$pr_id = 10;
$pr_id = sprintf("%03d", $pr_id);
echo $pr_id;

//outputs 010
-----------------

You can change %03d to %04d, etc.