Duplication in serial number while generating tickets - php

Question

I'm trying to generate barcode tickets after successful booking and it's working fine. But yesterday (06-Jan-2017) night at 11.59 pm, one customer booked 2 tickets but the ticket sl.no was duplicate. Tthat is both tickets sl.no was 1. Actually it should be 2. But the next customer's tickets sl.no was correct, it's 3,4,5. Is there any mistake in the code below where the tickets get generated?
for($j=1;$j<=$cat_array[$i];$j++) {
$sel_max_dt = mysql_query("select MAX(DT) AS maxdt, MAX(REC_NO) AS maxrec FROM tkt_barcode");
//to select max date and max rec no.
$row_max_dt = mysql_fetch_array($sel_max_dt);
$dt_max = $row_max_dt['maxdt'];
//$no_max = $row_max_dt['maxno'];
$recno = $row_max_dt['maxrec'];
if($dt_max == date("Y-m-d") && $recno == $TxnID) {
//if same date and same bkid barcode slno continues.
$sel_no = mysql_query("select MAX(BARCODE_SLNO) AS maxno FROM tkt_barcode WHERE REC_NO=".$TxnID);
$row_sel_no = mysql_fetch_array($sel_no);
$tkt_slno = $row_sel_no['maxno'];
$tkt_slno = $tkt_slno + 1;
}
if($dt_max == date("Y-m-d")&& $recno != $TxnID) {
//if same date and different bkid barcode slno continues.
$sel_no = mysql_query("select MAX(BARCODE_SLNO) AS maxno FROM tkt_barcode WHERE DT='$dt_max' ");
$row_sel_no = mysql_fetch_array($sel_no);
$tkt_slno = $row_sel_no['maxno'];
$tkt_slno = $tkt_slno + 1;
}
if($dt_max != date("Y-m-d") && $recno != $TxnID) {
//if not same date start barcode slno from 1.
$tkt_slno = 1;
}
$sel_sl_no = mysql_query("select MAX(SL_NO) AS slno FROM tkt_barcode WHERE REC_NO=".$TxnID);
$row_slno = mysql_fetch_array($sel_sl_no);
$slno = $row_slno['slno'];
$bartkt_slno = $slno + 1;
if($i==1 && $j>$count_promo_adult && $arr_edit["PROMO_CODE"]!=""){
//adult cat and total adult count > total promo adult count
$disc_adult = $adult_amt;
$discamt=$adult_amt - $disc_adult ; // discount amt
$rate=$adult_amt; //normal rate
$net_amt=$disc_adult; //discount rate
$pro_cd ="";
//promo code is null when total adult count > total promo adult count
} //end if
//string pading digits
$len_txn =strlen($TxnID); // bookid length
$pad = 6; //str_pad digits
if($len_txn > 5) {
if($len_txn==6) { $pad = $pad-1;}// if 6 digit
if($len_txn==7){ $pad = $pad-2;}// if 7 digit
}
$fullbar = $TxnID.str_pad($tkt_slno, $pad, 0, STR_PAD_LEFT).$clnd_date.$counter_no;
//barcode no
$fields = "BARCODE_SLNO,DT,FIN_YEAR,Counter_Code,Branch_cd,REC_NO,BARCODE,TICKET_TYPE,CATG_CD,AMT,DISC_AMT,SL_NO,NET_AMT,BCODE_CATG_SLNO,BCODE_SLNO_GEN,BCODE_SLNO_CATG_TOT,DAY_TYPE,Check_in_date,PROMO_CODE,TAX,S_TAX,conv_base_rate,conv_tax_rate";
$values = "'$tkt_slno','$booked_date','$fin_year','$counter_no','$branch','$TxnID','$fullbar','$cat_name_tkt','$i','$rate','$discamt','$bartkt_slno','$net_amt','$j','$count','$cat_tot','$type_day','$date_check','$pro_cd','$tax_each','$ser_tax_each','$conv_base','$conv_tax'";
$sql_tkt = $objA->insert_fields(TABLEPREFIX.'tkt_barcode',$fields,$values);
}//end for loop

Answer 1

You are selecting a count of existing records
$sel_sl_no = mysql_query("select MAX(SL_NO) AS slno FROM tkt_barcode WHERE REC_NO=".$TxnID);
$row_slno = mysql_fetch_array($sel_sl_no);
$slno = $row_slno['slno'];
$bartkt_slno = $slno + 1;
and using that count to set your sl number. But time passes between reading the count and writing the record. When two threads read the count at the same time you get your problem.

Answer 2

Finally I found the solution by adding another condition given below.
if($dt_max != date("Y-m-d")&& $recno == $TxnID) {
//if different date and same bkid barcode slno continues.
$sel_no = mysql_query("select MAX(BARCODE_SLNO) AS maxno FROM tkt_barcode WHERE DT='$dt_max' ");
$row_sel_no = mysql_fetch_array($sel_no);
$tkt_slno = $row_sel_no['maxno'];
$tkt_slno = $tkt_slno + 1;
}
Thanks guys for the help. I will try to make my code more readable next time. :)