<?php

    $rand = rand(00001, 99900);
    $difference = 5 - strlen($rand);
    if($difference) {
        for($i = 0; $i < $difference+1; $i++) {
            $cupom = str_pad($rand, strlen($rand)+$i, "0", STR_PAD_LEFT);
        }
    } else {
        $cupom = $rand;
    }

    echo $cupom;