kode unik (PHP)

//mencari kode terakhir
        $kd_reg='INV';
        $cari_kode=mysql_query("select tbayar_no_bukti from tb_transaksi_pembayaran
            JOIN tb_beban ON tb_transaksi_pembayaran.tbayar_beban_sud=tb_beban.id
            order by tb_transaksi_pembayaran.id DESC");
        $tm_cari=mysql_fetch_array($cari_kode);
        $kode=substr($tm_cari['tbayar_no_bukti'],3);
        $tambah=$kode + 1;
        if($tambah<10)
        {
            $tbayar_no_bukti=$kd_reg.'00000'.$tambah;
        }
        elseif($tambah>=10 AND $tambah<100)
        {
            $tbayar_no_bukti=$kd_reg.'0000'.$tambah;
        }
        elseif($tambah>=100 AND $tambah<1000)
        {
            $tbayar_no_bukti=$kd_reg.'000'.$tambah;
        }
        elseif($tambah>=1000 AND $tambah<10000)
        {
            $tbayar_no_bukti=$kd_reg.'00'.$tambah;
        }
        elseif($tambah>=10000 AND $tambah<100000)
        {
            $tbayar_no_bukti=$kd_reg.'0'.$tambah;
        }
        elseif($tambah>=100000)
        {
            $tbayar_no_bukti=$kd_reg.$tambah;
        }
        //akhir mencari kode