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.